How can I get legal custody of my exboyfriends dog?

I bought the dog for him and all papers are in his name, except a prescription card for dog food (I think it is in my name) but I am basically the caretaker. I paid for the dog, all his shots/health expenses, toys, treats, shampoo, medications, bones, pads, etc. He only paid for some food, pads and couple toys. Even when he had money for his stuff, I STILL paid, I clean his pad and feed him majority of the time. He sometimes does. I give him his medication majority of the time. He sometimes help. The only thing he does that I do not do is bathe him because it can be a struggle with me with the size of the dog. The vet put him on a special diet and he (my ex) has said himself that he does not care and that he will give him human food anyways even though the vet said not to. He also told me multiple times I can have the dog and didn't go through with his word to be mean. He is unemployed and I am employed. Please help me.