It's a tricky one when an 18 month old has not yet fully learned to speak. He will know who his Dad is if both parents tell him. If you're worried your ex will tell him her partner is his Dad, well you have legal recourse there. Keep records. Make notes. Keep all emails and texts. How do you find her new partner? Is he ok? Are things reasonably amicable? Just because he is there more doesn't mean he has the same level of relationship and no doubt he is out working a lot. Children adapt to situations. Yes your son may experience a "family" situation with your ex and partner, and a different situation with you but they are not comparable - it's about the love and bond with a parent, not just the situation. On a positive side, your son will look forward to seeing you because he sees you less. But I understand it's hard. In 6 months time, if your ex will not agree to more time, you could apply to court. Could you do 50/50? When you say one day a week - is that also one night a week? Nights are counted with previous time. However if it's 2 days and one night you could highlight that too.
It's normal to miss your child and feel something is missing. I found the best way was to keep busy preparing for when my son was coming next - by planning things to do with him, organising stuff at home for him (sorting his room, thinking what we would eat, where we would go), even reading child development websites and watching parenting programs on TV - anything that was child related that helped me feel connected when he wasn;t there.
Maybe you could keep amicably trying to suggest that a midweek overnight would benefit your son and give her a night off. But sadly many of these exes are "up" on the system and know that extra nights means less CM payments and means you're more likely to get a better order in future.
You don't necessarily need to wait till your son is two - it takes a few months to get to a final hearing. You could apply now for the same time as now plus a midweek visit or overnight, progressing to a full schedule when he is two and a term time schedule when he starts school age 4 - all in one order, You are likely to get a good result if you can show that you have bent over backwards to be amicable and reasonable and to resolve matters without court. Hence writing a polite friendly email with child focused proposals and attempting mediation before court.
The idea of a polite friendly email with your proposals is - even if it doesn't work - it's evidence for final hearing - what you tried to achieve, reasonably and what your ex's response was. If she doesn't respond by text or email but just verbally, then type out a diary note of what her response was - that can also be evidence.