Why do my fiance get boils on his behind frequently?

See a dermatologist: This kind of problem is common, usually a varient of acne. A small dose of daily Minocycline orally + topical benzoyl peroxide may clear it. Expect to continue treatment for about a year.

Heat and sweat: He may have a condition call hidradenitis. Keeping the skin clean and avoiding excess oil on the skin my help. Also try antibacterial or benzoyl peroxide cleanser. Just remember that BP cleansers do bleach fabric.