He ought to go see his pediatrician for followup. But you can try some
Hydrogen Peroxide antiseptic, just pour a little on a damp tissue and he should dab that area daily after he bathes. Then to prevent chaffing, he could put a little plain KY jelly on top, the same way. Probably a week, and he should see if it looks more healed up, less red, and any opening should be closed up. Then he can just put KY for maybe another few days, to keep it from reopening.