I cannot confirm anything without examination,but it can be folliculitis,seat dermatitis,heat rash,diaper rash
,contact dermatitis or psoriasis.Looking at the age of the child,diaper
or heat rash
is a possibility.
Allow your son to wear cotton undergarments and keep the buttocks clean and hygienic.Also after washing the area with water,pat dry the buttocks.Pls do not apply any cream on the byttocks as it will leda to further clogging of pores.You can however apply a light dusting powder.