I want to gain my weight and muscles. what exercises are best for this and how please explain friends.?

hi friends my brother wants to gain his weight and muscles. so please help him to know that he want to do heavy weight exercises or low weight exercises in gym like chest and shoulder and arm exercises. and tell what exactly weight he has to do . basically he is doing 90 kgs(kilograms) for chest and 30 kgs for shoulders. so please tell how much weight he has to lift i mean heavy weight or normal weight or low weight to gain his muscles. please explain clearly. thank you friends.