S.T.
Call your pediatrician. It's always your safest route. Doctors see rashes of all kinds, caused by so many things. They have the know-how to determine if it's dry bumps, heat rash, or an allergic reaction.
Before giving your son Benadryl or any other meds, consult your pediatrician. You'll be glad you did - even if it's nothing to really worry about. If it's more serious, you'll be happy you went in.
Good thing he's not itchy or irritated by it. That probably means it's not that big of a deal.
I say this because my daughter started getting bumps on her legs that looked like little bug bites. It went on for weeks and each week she'd get one or two more. It was strange - she had no other symptoms. We went to the doctor and found that it was a more serious situation, and she needed to go to a pediatric dermatologist. As a result, a few prescriptions and medications later, many weeks later, and two specialists later, we discovered she had a reaction to immunizations - a condition called Gianotti Crosti. It takes care of itself and doesn't need any medications. Just took a long time to get over. Of course she presented different symptoms than your son, but we only found this out by going to the pediatrician.
That's my advice to you. Hope you get the answers you're looking for. I am sure it's not a serious situation - your pediatrician can probably lay any worries to rest for you.