A friend of mine won't show up on Facebook.?

I've been looking for a friend of mine on Facebook, and I can't find him anywhere. Other friends I've talked to don't seem to have this problem. I've looked for him on their accounts, and I've found him every time, however, he won't even show up on my account, even looking for him on my other friends' friend lists. I'd hate to think that he had blocked me, but so far, that seems to be the most plausible situation. Does anyone know what might be wrong? Thanks.