How do you fix sagging breasts in men?

Gynecomastia : This depends if there is excess fat/breast tissue with or without loose skin. Usually this is treated with liposuction, and may require skin excision if not completely resolved with liposuction alone. If there is a tremendous amount of loose skin this may be addressed at the first surgery.

Depends in degree: For mild degrees of fullness and early sagging liposuction and it's various forms (ultrasonic, power assisted, radiofrequency, etc) may help to diminish the size and help skin contraction particularly in a young individual. More advanced forms will require surgical excision of excess skin which may leave more noticeable scars.