What Causes Bright Red Rash On Leg Spreading To Arm And Hands?

My son has a bright red rash that varies in different size bumps. It started on the tops of his feet last nigh and has since spread up both legs, undersides of his arms, both hands (palms and backs). There are a few on his stomach and back. Any ideas? They are raised and bright red with the surrounding skin bright white. They are not pussy or crusted.