It depends on cause. We need more details as to his age, what hormonal imbalance he has, how large are his breasts, is he obese and several other factors which can only be determined by a detailed history and physical exam and if needed blood tests. If he is of the age of puberty than it is possible he may not need any therapy so i will suggest check with your family doctor to get the right answer to your question.
Review medicines. Male breasts may enlarge secondary to some medications like spironolactone ( aldactone)or medications to treat prostate cancer ( Lupron or trelstar (triptorelin) injections, Proscar or finest arise . Anything that increases Estrogens or reduces androgens in a male will do it. Patients with liver disease, cirrhosis have it often. If there is asymmetry or a palpable mass pt. Will require a mammogram. Check meds.