Seems like your son has an excessive dry skin
which in turn leads to itching and scratching...
Or he might be a case of atopic dermatitis
.This is yet another causing development of eczematous lesions and itching and is seen mostly in young children...
You should start adequate moisturisation of the skin,,,apply oil like almond oil or olive oil over the body everytime after taking bath,,Use good moisturizing creams /lotions atleast 2-3 times over the body..He should wear cotton clothes as much as possible and use cleansers meant only for sensitive skin for taking bath...
All these will reduce the amount of itching over the body.
Now as far as the black marks and scars
are concerned,some degree of improvement will occur with prolonged usage of moisturising creams, but if they still persist, then you can apply a mild lightening agent containing kojic acid
and vitamin C
over the affected areas twice a day.
Hope it helps
Dr Geetika Paul