on gluteal skin is a common problem, and is caused by the same oil buildup and irritants that cause acne on other body parts such as the face, back, and upper chest. It is especially common for men to have acne in locations other than the face due to higher testosterone levels.
If he is using a lotion, soap, or bodywash that has additives or oils in it, he should switch to a noncomedogenic soap, such as neutrogena, for the gluteal area. Moisturizing shampoos and conditioners can also cause pores to clog when they are rinsed down the back, so he should wash his back and bottom after rinsing out his last hair product.
If he has predominantly inflamed pimples, the best treatment will be benzoyl peroxide. It can be purchased as a 10% body wash, which should be left on for 20 seconds before washing it off. There are also cream products which are applied and left on. Benzoyl peroxide can bleach fabric, so use with caution around clothing and linens.
If the lesions do not improve or he develops fevers, deep nodules, scaring, or other alarming symptoms, he should see a primary care physician
to determine if he would benefit from antibiotics or other treatments.