It seems that your son may be having tanning
effect due to sun. I would advise to cover the area with clothes to protect from sun. And sun screen may be used whenever he is exposed to sun. Give him lemon juice daily. Give him bath with kojic acid
containg soap. You may apply kojic acid and vitamin C
containing cream. Give him vitamin C and glutathion tablets for long time. I feel he may be improved.
I hope you or my answer.
Dr. Ilyas Patel MD