How long has Sonny been home again, it may be just that Max doesn't like a smell on him from the animal hospital and that is why he is hissing.

you could try buying some of those bath wipes and rubbing both of them so they smell the same or something?

 

I agree, Max is probably reacting more to the "scary" smell than anything. Cats are funny, they have a great sense of smell and can react very strongly based only on scent!

I remember a couple of years back we were fostering orphaned kittens. Our Shih Tzu (thinks he's a cat!) likes to mother kittens and they became very close. Well being spring the dog was due for a shave... when he returned from the groomers the kittens poofed themselves up, hissed, spit, and tried to attack the poor dog! It took a couple of days for them to remember him. Generally our cats don't recognize him right away when he gets his hair done but seeing six week old kittens out to 'get' him was rather funny (the dog wasn't so amused...).

I'm sure your kitties will settle down. Poor Sonny probably still smells like lots of scary medical stuff and other animals too. Even just rubbing him down with a blanket or bed the cats sleep on will probably make him smell much better (though you probably won't be able to tell what the big difference is!).

Feral Fan

 

It's very common for cats to think a best cat pal home from the vet or hospital is a stranger for a while. As the above posts said, they smell different. Just be patient, it'll probably be OK in a day.

 

Not only that they smell different, but scent is an excellent memory trigger. They remember that smell as being the place where they get touched by strangers, get shots and other things of that nature.