What is this?

Nadia • Mommy to beautiful twin boys, Dominic and Roman 💙💙

Have any of you mommies seen this? It’s on the back of my little mans neck. I sent a picture to my mom and she tried to say hives.. not really thinking it’s that. My boyfriend has looked it up and thinks it could be heat rash since he sweats a lot.

Just curious if I should call his doctor or if I shouldn’t be worried about it