Thanks for the query
You have not mentioned for how long he has developed this problem. Most of the time roughness and pigmentation over knees are due to constant friction or pressure over that part. You can use scrub and avoid friction over that part. Continue moisturiser application for one more week. If it persists even after that then consult a dermatologist
for proper clinical evaluation.
I hope this answers your query well
Wish you and your family good health