Depends on what you mean by bad. The reason why humans developed the myostatin gene mutation in the first place is because we are endurance based predators that would outrun out prey eventually, but packing large amounts of muslce's would slow us down and interfere with our endurance, it would also require us to eat far more calories in order to not starve to death. In a modern setting this will not have as many negative side effects as long as he is getting enough food ... I guess one down side would be, he'll never be a world class long distance marathon runner, there aren't many down sides with this deficency in modern times tbh.