If medicated shampoo and lotion have no effect this is probably not simple dry skin/dandruff.
Possibilities range from psoriasis to eczema and I would recommend a Dermatologist.
In the meantime I would simplify his skin regimine. For lotion and shampoo and laundry detergent I would change to allergen/perfume free. A loofah and mild soap for showers with warm (not hot or cold) water. Avoid hair products (hairspray, gel, conditioners) because they bulid on the skin and scalp and a gentle moisturizing shampoo is plenty of conditioning. Avoid hot tubs and swimming pools. You may try a mild antihistamine for a couple of weeks in case this is allergy related.
Hydrocortisone cream can be applied to especially scaley areas as well, though it is not a solution for full body.
If he has eczema or psoriasis there is prescription medication to address the problem. Since this is not only chronic, but also disrupts his daily life, I would see the Dermatologist ASAP.
I hope my information is helpful. If you have more questions, just ask! I will gladly help, otherwise please click "accept".
Positive feedback and any bonus would be appreciated. Thank you, Christy