It sounds like your son has slapped cheeks its a common virus with no symptoms should go away in 1-2 weeks,
My Son Has Red Cheeks For 3 Days..no Rash, No Fever, No Cold...what Is It?
My kids get it when I wash their sheets with certain detergents, higher priced ones usually, but generic detergent does not do this. We use flannel sheets and this seems to happen after they nap/sleep on clean sheets. It comes and goes. Could be this, or maybe lotions or soap too. My kids have sensitive skin lately and even hand soap has been a culprit.