I think my cat has a mental disorder. Help?

He is 7 weeks old, but still appears as a 4 week old kitten. I think it could have something to do with him being inbred, but he's the only one of my litter of 4 that has this problem. He just learned how to walk good, he doesn't have teeth, his ears aren't even near fully erect, and he still won't even eat soft food. He tries to play, but he seems to always trip over himself or run into things. He walks better backwards then he does forward. I'm not sure what to do about it, and I don't think he's old enough to go to the vet yet..