It is unlikely to be allergic reaction
You have not mentioned the age of your son.
Is he on diapers? If yes, it is a nappy rash- that occurs due to constant contact with urine/stools of the diaper. When you see white bumps, it might be due to secondary bacterial infection or fungal infection
Try to consult your Pediatrician
If bacterial infection, he will need ointments like Mupirocin to be applied 3 times a day for a week. If severe, he might need oral antibiotics
If just a fungal infection, he might need ointments like clotrimazole
for a week.
Keep the area open. Do not use diaper for a couple of weeks. Diaper creams will help by acting as a barrier between the skin and urine/stools. Do not wipe the area hard after he passes stools. Be gentle, just a mop with wet cloth will do.
Hope my answer helps you.