Is there any way to get legal ownership of a cat?

It seems like a dumb question but it’s actually a very serious issue for me. I am currently trying to get out of my house (my ex boyfriend is my roommate) and I would like to take my cat with me. Here is my problem.

4 years ago (when we were still dating) my ex’s cousins found the cat and gave it to him.

When his parents had to move out of state (he lived with them, with the cat) his parents put all 3 of their cats outside to fend for themselves as opposed to finding new homes for them.

My sister went to the house and found the cat, and left it with my other sister. Then I took the cat, paid for his food, etc. Then my ex moved in with me. I made him pay for the vet bills because he still claims the cat is “his,” so my ex’s name is on all of the cats vet records.

He does not take care of the cat. He has cleaned his litter box TWICE over the past 3 years and has fed him less than 10 times. My ex also bought himself a hamster, which he has never (I MEAN NEVER. Not exaggerating) fed her. He’s never cleaned her cage. So I have to do all of this for him now. I take care of the cat AND the hamster 100% of the time now. When I leave the house for too long, my cat (Ashes) stops eating and gets extremely depressed. He always sleeps in my room and he does not even like my ex for the most part.

THAT BEING SAID…I’m now planning to move out of state with my current boyfriend in the fall, and I would like to take the cat with me (he should adjust to change well, as he is still quite young) but my ex has been arguing with me about it. I know this is going to turn into something huge.

The cat doesn’t even like him. He often bites/attacks him! I know if I leave Ashes with my ex, he will not be taken care of properly. My ex works 24/7 and doesn’t know a single thing about taking care of animals. It seems like he just wants to keep Ashes out of spite.

I know a lot about pet care, and my cat is the center of my world. I take EXTREMELY good care of him. He’s my baby. How can I fight this? Is there a legal way to say “this cat is mine and you are unfit to take care of him?”