My 15 Year Old Son Has Red Pimple Looking Rash.

My 15 year old son has red pimple looking rash. In the crease in his underarm its a red patch similar looking to a yeast infection on skin. The red pimple bumps have been spreading down his side and up onto the underside of his arm. It's been slowly spreading, hydrocortisone cream seems to slow it down a bit and take some of the red out of it but it a not getting rid of it. Any ideas?