Hello,
Chicken pox scars are very tough to treat. Apply hydroquinone cream or creams containing vitamin E on the scars and it should go away in a few days to months.If it still persists then you can go in for laser treatment,photodermabrasion or microdermabrasion. Avoid picking, harsh scrubbing, and abrasive treatments unless prescribed by a dermatologist during this period.
I sincerely hope that helps.Take care and pls do keep me posted on how your son is doing or if you have any additional doubts.Kind regards.
http://www.ehow.com/how_4546502_fade-chicken-pox-scars.html
Website link above that might help