Thank you for your query,
With your description the rash
is looking like an allergic component as it is transient and on and off.
Try to find out any related allergen my carefully noting his diet, activity ,play with pets.
Consult your Pediatrician
whenever the child is having the rash for physical examination as the diagnosis will be easy.
Temporarily you can apply calamine
lotion for soothing effect .
Hope i have answered your query.