I learned from this site that son should not do weights until puberty hair was a thing. So son did a sport specific weight training program with a trainer starting the winter of freshman year. Trainer was always on site when he worked out.
As for gaining weight, it is still a struggle at 26 and playing professional baseball. I am glad he is off my meal plan.
nycdad is correct. Think peanut butter sandwiches right before bed, protein shakes, and double portion sizes.