He doesn't wear a gentle leader, just a collar.
I've tried to get pictures, but it's impossible. The bumps are skin coloured, so they don't show up. If I take a side shot, you can see that there is a bump under the hair, but not the bump itself. So far the bumps have only come on a Thursday or Friday, maybe next time they will come during the week so I can get him to the vet (not open on weekends and very busy).
I was thinking hives, but they don't seem to be itchy, and they aren't red. Though, I don't know if hives on dogs are red.
He was groomed a couple days ago. So maybe it's an allergic reaction to the soap the groomer used or a spray she may have put on him. He's had the bumps once before, and been to that groomer once before, but I don't remember if it was around the same time.