Unfortunately, you need to get a solicitor as well. I suggest you get one and get them talking. Mediation is the best option here but you shouldn't be making a decisions when he has one in place. Leave the child our of it as the others have advised.I don't have a solicitor. If I did, I wouldn't be asking questions here. I was trying to resolve our differences with civil conversation or mediation but he just wants to do everything via a solicitor.
I know it's not cheap - but its your child and will be worth it (there are cheap options) and it may mean that the other party will back down on their "requests". They have pushed the 50/50 for 5 months to create a precedence which they now have. Not the end, you have a case to push back, but I recommend you lawyer up sadly.