He's doing it to add weight because it sounds like his metabolism is really fast and not so much to rebuild muscle after a workout.

There are so many protein shakes out there at least a hundred if not closer to a 1000. Not all are created the same. Some are almost 100% protein while some are a combination of proteins, carbs, fats, vitamins, and minerals. You need to do your research when chosing which one you want. You want to read the nutritional guides and ingredient lists. Some are going to use quality ingredients others are not. Look at the types of fats used and the breakdown of essential and non essential amino acids (essential cannot be produced by the body).

One of my favorites is Met Rx. An excellent product but I used it more as a meal replacement rather than a supplement. I didn't want the calories from the shake and a meal which is more of what you are looking for. In moderation, they are for the most part safe to drink everyday. You know if you are drinking 5 a day made with whole milk then that might be a problem.