I don't see what the cats and the renting situation has to do with you wanting to end your relationship.... Is it his fault you have to move out? No? Then why put the blame on him and throw in the towel? Why are you living in a three bedroom house where I presume that he pays the majority of the rent and bills (and food for your cats) while you're still at University? Do you really think it will be easy for him to watch you swan off to your parents house, while he has to pick up the pieces of his old life and has no family support. The offhand way you say... 'he'll be fine he has a decent job' strikes me as particularly selfish and self-involved.
It may just be the way you wrote it.. But sounds like you're taking advantage of this guy and using him when you need him (money) but dropping when things get tough.
Things happen in life that people need to react to, if you're with someone for three years it's usually presumed that you will stick by them through stuff like having to downsize. It really doesn't seem like he did anything wrong in this situation unless you're dripfeeding... I think you're immature and spoiled. Do you even care about this guy at all or do you just think you're better than him because you come from a 'nice' family and have friends?
They may be vile (in your opinion) but they are still his family. The last thing anyone with a difficult family needs is a partner that constantly moans about how vile they are and constantly compares their family to your 'better' family. How unsupportive is that?
Have you stopped to think that maybe he doesn't make friends easily or go out much because of his upbringing? That being around parents that aren't the best might have had an impact on his personality in some way? That it made him who he is?
I'm sorry if this comes off as harsh but I feel so bad for your boyfriend of three years, neither his gf or his parents give a shit about him. Funny that you say his parents are selfish and then he's ended up in a relationship with someone like you. He could be with someone else who would actually nuture and support him emotionally, rather than judging him because he doesn't want to go out and get pissed. (I presume because he has a 'decent' job and has to work to keep a 3 bed house over your head... Two cats ain't cheap either!)
I think you should suck it up and leave him because that's clearly what you want to do but you're only staying for external reasons.i.e nice house, space for cats, feeling like the 'better' person. He deserves better! Stop toying with him so he can find someone else, better suited and less selfish.
While you're at parents, why dont you get a part-time job so you can start saving up for your own place? Thats how you get the independence you so sorely want.
I've name changed because I feel like this is against my normal posting style by being harsh but someone needs to tell you how you're coming off, so you can take that into your thinking and look at the situation from a perspective other than your own.