You definitely need a court order if mediation doesn't work. Good that you have all this to show how often you had your son with you, just in case it's needed. It probably won't be because you can say to Cafcass - just ask the school how often I was there to pick him up. Six weeks away is far too long for a child that age. But - in times of Covid all kinds of things happen and you may not be able to prove she was making that up. Or there may actually have been travel restrictions in place possibly.
She will have needed an exceptional reason not to be fined by the school for the absence, but maybe they just believed her and primary schools are not quite so strict about absence for holidays.
With a Child Arrangements order, a parent with a "lives with" order can leave the country for up to a month, but no longer without court permission. Without a court order then technically both parents need the consent of the other to go on holiday. If your ex has "lives with" then you would need her consent to go abroad (even if the court order designates holiday weeks with you). If it's "lives with both parents" either of you can go abroad on holiday without needing the consent of the other. And the "up to a month" has to be within your own court ordered time, or by agreement with the other parent if it's outside of that. Eg an exchange of dates.
I can imagine your son is distressed with this sudden change of routine when he has spent so much time with you so I would absolutely apply for a Child Arrangements order. You will still need to try mediation first but it sounds like it's not likely to work if she won't even agree to you taking him on holiday.
As you're both from countries outside the Uk then overseas holidays are presumably an important thing - but also possibly a risk causing anxiety to the other parent (works both ways) - eg fear they may leave the country and not come back. However, with the Child Arrangements order in place, if either of you did that it would be abduction and they could force the return of the child (depending which country it is). Which is a deterrant in itself and the parent would probably lose "residency" (ie the child only "live with" the other parent and "spend time with" the one who had tried to abduct.
There's also a situation where, if you think there is a risk she may suddenly leave the country, you can apply for a prohibited steps order to prevent that, in advance. But think carefully about that, as that could work both ways. Do you think she is likely to leave the Uk permanently? Some countries are outside the Uk jurisdiction, but I'm not sure which ones off the top of my head. Poland won't be but I don't know about the Dominican Republic - Invisible Intellectual might know about that.
My son also started regressing with wetting himself at that age after sudden changes (my ex moved house and changed his school) although I was still seeing him, but he did get upset about it. It will be worse if he also has the stress of a sudden change of routine with a parent and seeing them a lot less.
Start filling in your C100 now so it's ready to send as soon as you've had mediation - that's a time to act quickly as your ex may have a good idea you'll be applying to court if mediation doesn't work/happen and my view is - best to get in first. Then if she did accuse all sorts of reasons for reducing time, it's fairly clear it's retaliation.
As you'll be applying for "lives with both parents/shared care" then use the paper form, not the online one. The online application doesn't give the option to put home addresses. It just says - where does the child live" - and that means most Dads put the Mother's address - which automatically gives her a residence/lives with order! When there are no orders in place, neither of you has "residency" - you are equal - with equal parental responsibility. However Mum's behave as if they have sole residency sometimes and possession is 9/10ths of the law.
On the paper form you can put "Child lives with both parents" and put both addresses. Because that was the case until she unilaterally decided to change it. It's a bit more tedious as you need 3 copies of everything to take to court, but it's worth it to ensure you don't accidentally give her a residence order.
There's a guide under the tab above "Legal Resources" - how to complete the C100. It's not finished! But takes you part way through and ask on here if you're stuck about what box to tick on occasion. An important section is 5b where it asks why you're applying. The box isn't that big so I put "please see attached sheet" and type it out/print it out on A4 and attach that behind that page of the form. Which is fine. No more than a side of A4 ideally or it might not get read and hence needs wording carefully and to read easily and give a clear picture of the situation, the history and the issues. At the end of that you put what you ask the court to order. So you would say something like.
I respectfully request the court to make an order as follows:
For my son to live with both his parents, shared care, on a 2-2-5-5 basis. With every Wednesday and Thursday night with me, every Monday and Tuesday night with his Mother, every other week-end with each parent, and half the school holidays with each parent. I request that I retain child's passport but make it available to the Mother within 21 days of any booked holiday.
At a later stage in the process you can provide much more detail for what you actually want in an order. The more detailed, the better, to prevent ambiguities or loopholes. Eg times the days start and end, when school holiday periods start and end. And ideally a clause that says neither parent will change the child's school without the consent of both parents.
What that last bit does is help avoid any sudden long distance move - you get pre warning if child's school is told Mum has given notice. So you can then do an urgent application to prevent child being moved away. That is foresight as that is sometimes what an ex tries to do, once they are tied down by a court order.
So have your C100 filled out, photocopied, ready to take in/post as soon as you're signed off mediation. If you don't need it, fine - it's been a useful exercise in case you ever do need it.
The paper form is the second download on this page. You can type it on a computer, edit it and save it/re-edit it. Then when complete, print it out.
I think it's page 9 that is the form for the mediator to sign. So if you're sure your ex definitely won't go to mediation (if you've suggested it) - you could take that page from the form with you to mediation and ask to be signed off there and then and tell them it's urgent you get the application in as soon as possible as your son is under stress. They might still want you to wait a couple of weeks while they invite your ex and give her the opportunity to attend - but that's up to you. You can mention in your application that she declined to attend (if she does).
Use this form to apply for a 'child arrangements', 'prohibited steps' or 'specific issue' order under the Children Act 1989. You might be able to apply online.
www.gov.uk
We thought a guide to completing the form could be useful. This is the C100 form for applying for a Child Arrangements Order, Specific Issues Order or Prohibited Steps Order. C100 form on .Gov website If you scroll down the page you will see...
www.dadswithkids.co.uk