Cat losing large amounts of hair?

My black cat, Apollo, is having some serious fur issues. In the last four months he's been losing fur like crazy. It's bad enough that there's a significantly large bald patch on the back of his hind legs and a few other smaller bald spots in other places. My husband and I can't figure out why he's lost so much fur, except that he does sort of get picked on by his brother and sister. They don't wound him, but I'm always having to pick up tufts of black fur after their tussles. And we wonder if it has something to do with his being out missing for two and a half weeks (this happened late December/early January), since it seems to have started just after he came home. He was skinny when he came back, but he plumped up nicely right away with no puking or other problems. Does anyone have any ideas what might be going on?