I havent read all the posts so sorry if i repeat anything.
The best thing to do is try and cut out external irritants, not change diet. Things like biological washing powder, cleaning products and soap or handwash can make it worse.
Bath him everyday using only an emollient in the bath, like Oliatum. Also use this to wash hands.
Then after the bath dry his skin really well and cover all the affected areas in the cream, it doesnt really matter which one, Diprobase and Dermol are good.
Put the cream on a few times a day, but wash the skin with water and bath emoillient first and wash your own hands before as well.
This should not be too often but when it is really bad and the above is not helping it is ok to use the hydrocortisone very thinly on the affected areas. Use this for up to 5 days at a time.
I had the same with my boy, this advice helped us. He is now nearly 5 and has pretty much grown out of it, only getting a flare up very rarely.