This is his arm n he’s broken out under her chin n his neck we didnt switch detergents... eczema???

Looks like eczema to me . My daughter literally has the same thing in the same spot . I just rub some vaseline or aquaphor on it . Dr said to keep it moist with something like vaseline based. NO LOTION

Does he get sweaty in those parts?

Keep Aquaphor on it.

Heat rash?

I used to get this bad when I was a kid. Folds of my arms and behind my knees.