I'm not sure, but maybe he just can't cleanse the area because it's out of reach. You could wash his back or use tea tree oil on the area to keep the pores clean. Maybe that will help.
It can be due to blackheads or sebaceous cysts. Other than that it can be Pili multigeminis which is a rare condition in which separate hair follicle bulbs combine together into one hair canal towards the skin surface. Not many treatment methods are available however, Cryosurgery has been suggested to remove the follicle(s) entirely.
It needs to be examined and biopsied from a dermatologist to confirm the diagnosis.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.