There are lots of reasons why this occurs, but resulting in a nasty sore - states he should be seen by a veterinarian very very soon. Cats do heal well and fast, but often times the skin heals faster than the tissue below resulting in abscesses. Please take your cat to your veterinarian for an exam and diagnostics and treatment.
Although there are lots of causes, fleas is a very common one. Please be aware fleas can hide easily. Please also be aware that the over the counter flea products are not enough to stop the problem. It's like using an umbrella with holes in it in the rain. You'll get some help, but you'll still get wet. Often these can be toxic to cats and not safe for open wounds. See your veterinarian for the best treatment and if bloodwork is needed, it can also be a big help - even if it's normal.
Your veterinarian has some prescription itch meds that can help too (may contain small amounts of steroid) - these may not be necessary.
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