K.M.
I would probably say that is heat rash, and its popping up where he is resting and therefore sweating the most. My neighbor has a baby boy the same age and he has it on the back of his neck, its so hot and since he doesnt move around too much on his own he gets the little bumps.
I might try the Baby Aveeno lotion. I used that with my son when he was a baby and it worked really well.