I have a similar rash. Mine started on my left foot in small red spots that are raised to the touch, I then developed larger scabby spots and the rash has spread to both legs and up to my stomach. It is not itchy or painful, unless I catch the scabs. Mine too looks like chicken pox in places, however is highly unlikely due to it not being itchy. My doctor thinks it is scabies, however, I disagree as it is working the wrong way up my body and I have no entry wounds. Maybe look at his friends and at school and see if anyone else has any sort of similar rash as scabies is contagious. Another possibility is follucitis which is infected hair follicles. This usually clears up on its own, and this is what I have diagnoses mine as. I am currently treating it with tea tree oil cream as it is a good antiseptic/anti-fungal/ anti bacterial.
Heat rash is a possibility. Or a change in detergent (washing his clothes).... My son has a rash over his back, chest and arms. I'm searching the internet for an answer too!