I am sure it was stress! I hope he is okay now. I can bet he was just having anxiety issues and fear induced vomiting.
Well I know if I was attacked by three dogs and I were a cat (even as a human) , I would be doing more than throwing up!!!! You are lucky if that is all he is doing.
Just to be on the safe side try and get him to an emergency Animal clinic--they will usually work out some form of payment and if they just check for any injuries and there are none, they may not even charge you.
Also see if you can get a Feli-way diffuser for the room he is in--it will help calm him down for now. Poor little guy---you will have to lock the doors and throw away the keys--although after this experience you may never have to worry about him wanting to get out. He is neutered , right? That will also make him less likely to want to "cruise"--know what I mean??? LOL