How to find skin doctor for boy with moles where the sun don't shine?

Genital moles. Any dermatologist that sees pediatric patients can look at his moles in the genital area and tell you if a biopsy of any of the moles is needed.
Start with your prim. Your pediatrician or primary care doctor is a good place to start. They will refer you if necessary, you can also call a dermatologist or plastic surgen.