This could be due to sweat dermatitis or even jock itch - due to tinea infection. You should keep the skin
clean and dry, avoid clothing that rubs and irritates the area. You need to wash the areas several times with
fresh water. Do not use any cosmetic products at the sites. Apply calamine lotion at the site of the lesions
and see if it helps. You could take some oral antihistamine medications like cetrizine or loratadine. You need
to maintain a good personal hygiene. Wear lose comfortable clothes. Application of topical over-the-counter
antifungal or drying powders, such as those that contain miconazole, clotrimazole, or tolnaftate would help.
If the symptoms persist - it would be best that he consult a doctor. Regards.