My son has been diagnosed with foliculitis and has been taking antibiotics for two years with minimal results. What else he can he do?

Other cause. Depending on where the rash is located, it could be something else other than folliculitis. I would recommend a second opinion and possibly a referral to a dermatologist for further evaluation.
Location? Folliculitis could involve any body surface. Outer arms & upper thighs, neck & face are common. If prolonged oral meds have been used without satisfactory improvement, u should look into other skin care rx. Various topical abx or agents may be helpful. A review of aggrivating exposures or activities with your dr may help refine your rx.