I cannot really tell what the spot looks like from the photo, but I think it may be similar to a spot that appeared on the top of one of Willow's front paws. I never saw her chewing on the paw or anything out of the ordinary. I did not know what to do - and vet visits are so stressful, so I feel you on that - I was gently cleaning it with a warm, wet rag and applying a little Tritop each day (had it left from a previous issue). It would sometimes see it and think it was getting better, but other times, it looked bad again. Then, she also started getting bumps on her neck. At first, I thought maybe she had been itchy and was scratching herself, but I was never seeing her scratch excessively, and there were several of the bumps. So, I took her to the vet who said these things all appeared to be an allergic response. She said the little bumps can come out from within (vs being a scratch scab like I had thought). Willow has had issues with allergies for many, many years, sometimes, I think she is allergic to air

. These symptoms were completely new though. The vet said allergies can present in many ways, and that sometimes, even when a cat is taking an antihistamine (Willow takes Zyrtec daily), they can have a particularly bad allergy period where the allergy responses exceed the antihistamine's ability to control them. So, she suggested Temaril P which is an antihistamine (trimeprazine) and corticosteroid (prednisolone). I am paranoid about steroids, but she said the dose was so small that I should not be concerned about it. Also, I felt we were at a point where she needed it, and the rewards would outweigh the risks. The dosing was tapered; if I remember correctly, it started with 1/2 tablet twice a day for 4 days, then dropped to 1/4 tablet twice a day for four days, then to one 1/4 tablet a day for 4 days, then 1/4 tablet every other day for 4 final doses...something like that. Anyway, I started seeing improvement in the first couple of days. The round of Temaril P did it... the bumps went away, and the spot on her paw did as well, you could see/feel the soft little hair coming back in.
Does he have any history of allergies? Is that spot the only spot where anything is out of the ordinary on him? This may not be the problem in his case, but it's just another possibility to consider. If it was injection site related, wouldn't it most likely be occurring at multiple spots, assuming you do vary where you give the shot?