My son keeps a strange "rash" of tiny red bumps above his groin area in diaper?

it doesnt seem to bother him, but it looks horrible. i change him often, bathe him nightly, slather on a&d per urologists directive, use the nystatin morning and night, all to no avail. could this be sensitivity to his diapers (we use huggies)? are there diapers out there made for sensitive skin what kind and where to get them? has anyone else had a problem with this? any advice would be appreciated and thanks in advance.