assuming he doesn't have a skin yeast infection hiding in this warm moist area the a&d desitin or vaseline is just fine to use as a barrier against future moisture. you can treat this area just like a diaper rash.
my 4 month old has red raw skin around her neck probably from moisture - what can we put on it? vaseline a & d ? . she has been drooling we think the start of teething and try to keep her neck dry but it is very red and raw today after her nap. mostly in the creases.