Yes I agree with your son’s dermatologist that since the mole has come out as atypical non cancerous, hence it needs to be explored more and any melanoma needs to be ruled out. Another possibility that needs to be ruled out is of a recurrent nevus. Recurrent nevus is a melantic skin lesion which clinically resembles a superficial spreading melanoma at the site of a recent shave removal of a nevus. What happens is that melanocytes left behind in the wound regrow in an abnormal pattern. This is accompanied by scarring, inflammation, and blood vessel changes and this both the clinical and histologic impression of a melanoma.
I suggest you to get a punch biopsy skin done to confirm the diagnosis. I hope it helps. Take care and regards.