How can I treat a male genital rash?

Lotrimin (clotrimazole) It will treat fungus and yeast. Also use diaper cream to protect sensitive skin. If it persists see a doctor for more definitive treatment.
See your doctor. It depends on what is causing it. If you mean a jog itch type of rash, then an antifungal cream may help. Even more important, something to reduce the sweating in that area.