S.B.
Are you certain he isn't suffering from pinworms? Those can cause itchiness around the rectum. Especially if it's worse in the evenings, that would be my guess as to the culprit. When my neice got them, my SIL didn't see ANY signs. Her daughter just had periods of itchiness. She went to the doctor thinking it was a yeast infection. The doctor suspected pin worms and had them do a tape test (basically she put clear tape on her daugter after she fell asleep and they took the sample in to the doctor, who found eggs on the tape.).
My second guess is he is simply not wiping well enough or sweating and irritating the area. I'd have some wet wipes available and try that route to see if he gets some relief.