My Son Has A Rash And He Itches All Over

My son has a rash and he itches all over his body. A dermatologist says it is eczema and he does have patches similar to eczema but he itches even where he has no rash. I feel it's an allergic reaction to something, but we have changed soaps, detergents, etc and so far he still itches. any ideas? My son is 23