From what you say, it sounds very much as if his aim is to frighten you.
A Dad who was motivated by wanting to see his son more would have wanted more than one night a week a long time ago. Asked for whole weekends. Etc.
I agree, in theory there is nothing wrong - and a lot right- with him wanting more contact, he SHOULD have more contact - but spidey senses say that it's more likely this is intended to upset you, so whatever it is you disagree about now, you'll back down.
Given that - I would test the water. How directly can you communicate? Something like:
'If we are not able to do mediation then if you could contact the court and make your application that would be good so that we can get this sorted out. It would be good for X to see more of you and definitely to have more overnight contact, especially full weekends. It would also be good for me to be able to have more defined time of my own with a better schedule that neither parent can change, and I am sure that it would help our communication if the 'hard parts' of the parenting were more equally shared. Let me know when you have contacted court.'
Note what the above doesn't say:
- it doesn't mention 50:50, so he can't use it as you 'agreeing' or even considering 50:50.
- it doesn't mention money so he can't quote you on anything to do with maintenance.
It DOES let him know that:
- you're not scared of him taking you to court
- if he does, and gets more contact, it will mean him doing more of the work and, as a result of that, giving YOU more free time.
-it hints at him no longer being able to choose when he's free to see his son. Contact schedule is just that - schedule.
He'll be the one finding he's not free for the party on Saturday as that's his weekend and it's laid down in the contact order.
The above also shows any external person reading that you are supportive of more contact and a reasonable person.
If your Ex genuinely wants more contact, he will hopefully respond to it positively.
If he's trying to scare you by threatening court but actually the last thing he wants is to give you more time off by sharing the parenting more easily, it will scare him RIGHT off.