What suggestions is there for a scrawny/skinny male in his 20s to increase overall body mass, bone density, and strong athletic physique?

Weight training. . . To gain weight, one has to consume more calories than he is burning. Do this in a healthy way by avoiding junk or fast food. Keep a well balanced diet and make sure you are getting all the nutrients you need. Don't cut out exercise as it is very good for you. Weight lifting and other muscle building exercises can help increase one's body mass (muscles). Body-building "supplements" are not needed.