I hate my ex

My ex and I have a 3 month old and I'm with my baby all the time. My ex only gets him when he feels like it and I'm so stressed out! I have 2 other boys from a previous relationship and my baby is starting to teeth and is whiney most days. I'm really starting to despise my ex because he gets to do whatever he wants without taking on any of the work of being a parent to our son. My ex has a daughter from a previous marriage who he gets every other week but yet can't even come see our son on his weekends off? It's really making me feel like our son isn't as important as his daughter. He always has an excuse! I'm going to give him an ultimatum that he either starts taking our son or I'm going for child support because this is bs! He's gonna take on the responsibility one way or another bc this isn't fair to our son!