Tiny bumps on sons skin - Allergies???

My son (5 1/2), who I think has ADHD, has had tiny bumps on his skin for several years. They are pretty much invisible to the eyes. You have to rub your hand on his skin to know they are there. They are never red nor big. They are kind of dry. His skin just feel kind of gritty.

He always wants me to scratch his back. I've thought in the past that they were from a moldy mobile home we lived in, but since moving into our new mold-free home, they really haven't gotten better.

I was wondering if they could be allergy related instead.