Hi Meg.
Flesh wounds on cats heal very rapidly from the outside inward, which traps bacteria inside, causing cysts that fill with pus. Treat the wound with a 50-50% water/hydrogen peroxide blend applied with Q-tips or cotton balls to soften any formed scab, then remove the scab and irrigate with the 50/50 blend. If there already is pus present, cut back the fur with scissors, apply a warm compress to the area, then after about 5 minutes, massage around the wound, gently squeezing to extract puss and prevent a sub-dermal cyst from forming.
Be careful with vitamin C as sometimes it can loosen the stools of our furry friends. If your boy eats wet food/canned tuna, you can see if he tolerates fresh minced garlic cloves mixed in with olive oil (natural antibiotic). It is likely that the veterinarian will prescribe antibiotics. If, by chance, you have human antibiotics in your medicine cabinet, these can also be administered in wet food/tuna. The dosing would be about 1/4 the amount for a human. You can do this by grinding the tablet to a powder and mixing 1/4 of the ground tablet into the wet food/tuna. Start in the morning and observe for any digestive reactions. If your boy tolerates it well, a second dose at dinner can be administered.
If he is licking the wound a lot, you can apply some tea tree/lavender essential oil around the area to deter him. Also, see if any friends with cats have an old plastic head cone you can borrow.
Best wishes to you both, Meow!