Look to the future. You do have some growing up to do yourself.
Your parents are young enough to be my children 😶 , you have a good job, hopefully mat leave in place, a welcoming home and support, and a baby who'll be wanted and loved by at least three people. So many new mums and babies have none of that.
Your ex and his parents have zero power over you. They will miss out on the joy of a child and grandchild, which they may bitterly regret one day.
What he does have is responsibilities.
They will fight it, but he will be liable for child maintenance.
Personally, I'd keep everything businesslike, polite - and in writing. Avoid calls and texts, use email.
And I'd offer access, meetings, the opportunity for them to get to know their child/grandchild.
Be prepared for nasty responses (and keep them, should they try anything sneaky in future).
All the best for you, your baby and your brilliant parents.