Help please!

I just noticed this on his stomach. He’s also been having diarrhea. He was just at the vet for surgery for getting into a fight with another cat. He was checked for everything they said he was okay afterwards. 

Hello - and I'm so sorry to read that your cat has a wound.  I dont know the timescales in this case but it may be that this wound was inflicted during a cat-fight, either by a tooth or by nail.  Cat canine-holes are tiny and extremely difficult to feel by touch in a wide-awake patient when they first happen, especially given that many cats are able to hide pain so expertly.  In the vast majority of cat-cat aggressive incidents, shaving the patient is needed to find a wound, but also has costs to the patient - this would often have to be done under GA.  The weighing-up of this decision (whether or not to search for a wound) should be carried out by your vet, ideally with your awareness - it may be documented in the notes.  Now that this wound has been found, your vet needs to be aware of it as further treatment may be necessary, so please do ask for the case to be triaged again as soon as possible, ie now, for example by them or your replacement-on-call- emergency vet.