J.M.
My DD gets heat rash really bad when we put her in pull-ups or diapers. It looked JUST like what you just described...
My pediatrician recommended using a corn-starch baby powder to help absorb the sweat. (Heat rash forms when sweat isn't able to come through the pores... It's especially bad in young kids because their pores are too small to handle the amount of sweat... the problem is compounded when wearing tight clothes that hold the sweat against the skin...) If possible, you could try putting him in underwear to help that area 'breathe' a bit better. My DD is an excessive sweater, so she is especially prone to heat rash. Last year, I started putting her in underwear with one of those incontinence 'pads' instead of diapers. It helped SOOO much with her rashes! (Plus, it's cheaper. lol.)