I don't want to pay half of my son's daycare bill
Why don't you want to pay half of it?
I'm planning to leave his mom. She can take care of him by herself.
You have an obligation to pay half of the bill if you agreed to do so. It would be very selfish to stick her with the full bill.
Fuck that. I'm leaving her and I won't pay a penny.
Even if you leave her, the government can obligate you to pay your half of your son's expenses. If that happens, you may end up paying a lot more than just half of the child care bill. Your life will be easier if you try to have an amicable relationship with your son's mother.