Wow - that's pretty big and nasty.

I'm not sure that I would wait long to go to the vet with that. Is it getting bigger? Does it look like it's coming to a head?

I'm sure you normally don't take his temperature so you don't know what his average is, or I'd say take his temp.

I don't know what vets usually do, but when we have people come in with abscesses that size they lance them and put a drain in (just something to keep it open so all the puss will come out) and put a dressing over it. Then they keep checking and changing the dressings until it's time to pull the drain.

That's assuming it's an abscess. It's pretty hard to tell from your photo - without feeling it etc.