What is this? My babies now on antihistamines

Any ideas what this rash is? My poor baby. I took him to the emergency doctors last Mon and he said it's likely an allergy but it's hard to determine what to. He gave him an antihistamine to take (piriton) twice daily and he's going back next week to see him again. Even with the antihistamine he's still getting new spots and those spots have broken out today on his face. It's not as red and angry looking now. I don't use any products on his skin. He's bathed every 3rd night just now in just water and everything I use is non bio. His skin didn't start playing up until he was 2p days old and nothing changed. He's breastfed with formula top ups and has been since he was born.

I really hope it's not a milk allergy.