I have seen this a few times myself. It seems random and usually clears up without any action. It might just be a sensitive spot from the injection. Still, it is worth watching him for any further signs of allergy or infection since everyone reacts differently.
My husband and I have been using the GHK-cu and KPV shots for more than two weeks, though we skipped a couple of nights. Last night the same vial gave him a raised rash that feels hard underneath. It itched so he used cream and took Benadryl just in case. He skipped tonight. His allergies have been active lately and he has been itchier overall. I did not filter the first vials but I filtered the newest one.