Since it goes and comes and there is no fever or any other symptoms, it sounds like he is reacting to something- it could be either food, clothing that is touching his skin, or the type of soap you are using. Try to think back to same day/day before the breakouts.
It would be best to start a journal- the determatologist will probably ask you to do it anyway- day by day, bath with what, detergent used to wash his clothes, any sprays you might use (Febreze, etc), foods/snacks/treats/drinks -
everything that goes in or touches his body.
He is too young to do allergy testing, so you are going to have to start eliminating one by one to see what is causing the rash. It sounds complicated, but once you start the journal its not that bad, just keep a small notebook on hand and jot things down, this way when he breaks out you can see what he was exposed to before the breakout and then start eliminating those things one by one to find the source.
My son used to get a rash on his body also when he was about that age and he was allergic to Citrus(orange, pineapples, etc.) I figured it out by eliminating things one by one. Now he's 4 and he can drink/eat citrus again without a problem. Sometimes it is a stage the body goes through.
In the meantime you could try oatmeal baths, they sell it in the baby aisle, its to sooth rashs, that helped calm the discomfort for my son.
Best of luck to you!
A.