I actually would say that despite what people say on here, other people can and do change, however, only when they want to or they realise they can not have what they want without changing.
Right now, you're bitching about stuff, but still going along with what he wants, he says what he wants and he gets it, you dont like it, but you don't force him to change.
were you renting on your own privately before? So you can afford it, go find another property to rent, move out, tell him you won't marry him or be with him under the current arrangement. He can move in with you, minus the crap and the cat will just have to cope. If any more broken crap comes into your house, you'll ask him to leave.
However, what you have to be prepared for is when faced with a straight choice, the cat and his crap might well be more important to him than you and so he does'nt change and doesn't come running after you. Personally, being less important than the cat would be a deal breaker for me anyway, so if he would put the cat first, I wouldn't want him. You might feel differently, so you don't follow what is a high risk strategy if you want to be with him even ifyou are less loved than the cat n' crap.