Help! My boyfriend has man boobs. What can he do?

Surgery. Male breast enlargement is usually of unknown cause and the best treatment is surgical. This is generally a combination of liposuction (ultrasonic, in my hands) and direct removal of breast tissue that cannot be suctioned out. He should be within 10-15 pounds or so of his target weight. This is an outpatient procedure with minimal scars and very gratifying results. See a plastic surgeon.
Stop smoking pot! Lots of things can contribute to 'man boobs' aka gynecomastia. Pot, muscle building supplements and steroids, certain medications and even testicular cancer can do it. Have him checked by his regular doctor and possibly by an endocrinologist to be sure.