A good, sound progressive strength training program will go a long way for helping people of all body types add muscle mass. Of course, everyone has different genetic potentials, and yes, your client will definitely have to fight harder to gain lean mass based with his body type. But as the saying goes, “it is what it is.” I don’t know where he’s at in his training, but I would be emphasizing the basic, harder core strength movements such as squats, presses (bench & shoulder), rows , deadlifts etc. As far as his diet, he will need to increase his calorie intake if he’s looking to gain weight. I would suggest that the increase in calories come from protein sources, but I would need more information there. You don’t want to go overboard on the calorie increase – 200/day is a good starting point. I hope that helps Michele.