A couple of weeks prior, I had made the point that, in retrospect, they are my cats. I paid/pay for all of their vet visits, vaccines, food, litter, and monthly flea treatments. They are also microchipped with my contact information incase they ever escape. I had said this because if we were to break up, the cats would come with me (all of their paperwork has my name on it). My boyfriend chose to not financially contribute and I was fine with that.

Apparently this hurt his feelings, and he used that against me by not cleaning up their throw up or litter boxes over the past couple of weeks.

It sounds like OP openly stated a few weeks ago to her BF that the cats are hers in case they break up.

OP how did you think your BF would react when you discussed who would get the cats if you break up?

Is the BF being petty by not cleaning up ... Yup! But did OP choose to get kittens while going to school and working and choose to move in with a man who has his child on the weekends ... Yup!