Sounds like it could be an abcess. Do you think you could make the
appointment earlier to get it taken care of? Sometimes abcesses can
get really nasty and require alot of post care. He will probably need
the wound clipped and cleaned and if it's bad enough, they could have
to put a drain in it to be flushed out periodically.

Will the cat let you handle him enough to the point that he will at
least let you clean the wound until you can get to the vet?