URGENT - Please help my baby

I have a 5-year old cat, he's always been perfectly healthy if a
little disturbed by any changes he might experience psychologically
(he moved a lot before he came to us as his previous owner left the
country and he didn't settle anywhere else and it took a while to
settle here).

Lately he's taken to hiding (his usual behaviour when something
frightens him), which was not surprising since he came in from outside
one day in a state of shock and hasn't tried to go out since (he is
normally an indoor cat but we occassionally let him into the front
yard when we're about during the day).
Unfortunately we found him in one of his hiding places last night
looking listless and lying in his own sick. We gave him a bath and put
him in his favourite raised house next to the bed and looked after
him, making sure he is eating and drinking water.

He has been eating but he is incredibly listless and there seems to be
some kind of liquid (with a crackling sound) just under his skin on
his back. I'm very worried but we can't get to a vet tonight.

Please help if you know anything that might help us.