What Do Small Bumps On The Shoulder Blades Indicate?

My 12 year old son presented a concern about a skin rash/bumpiness two nights ago. I observed small bumps along his shoulder blades and the area between the shoulders. He is currently playing modified football and our temps here have been abnormally high (mid 80s). I suspected the irritation is related to wearing heavy football pads, sweating, and possibly hormones. I advised him to be diligent in hygiene (be sure you use SOAP in the shower, etc. This evening, my son showed me the rash again. He now has the bumpy rash all over his back, his sides, his chest, shoulders front and back, and abdomen. There are three red pimples. I immediately thought it was either an allergic dermatitis or a viral illness. Since he had already showered, I rubbed hydrocortisone creme on the spots. ???