It is my stance on the subject of using diet to achieve any fitness outcome to refer clients to a registered dietician. I will discuss recent findings in regard to diet and weight loss or performance goals. But in terms of weight gain, professional guidance will be more helpful. Simply consuming calories for the sole objective of weight gain can have numerous other undesireable outcomes. And the use of supplements is even more concerning to me as the actual content is not well regulated. And a number of other factors are involved in the development of each individual
You ask the question of safety and effectiveness for a particular type of nutritional intervention.
There are several questions here: is it effective, is it safe, is it effective and safe for the type of individual with whom I am working, and is it safe and effective for the specific individual with whom I am working.
People who work in fitness will have a variety of opinions on the safety and efficacy of various supplements, nutritional strategies, diets, fasts, juicing, etc…. But what they use themselves, and what they are ready to recommend professionally are not always the same. I suspect a majority of people, especially if they are not engaged in selling the products, would be for a whole, well balanced diet first, in any case.
The question you did not ask, but which will be part of every answer you get, is whether it is within the scope of practice of someone who is not a nutritionist to answer this safely and effectively. Suppose you advise this person to use a product like this and they have an allergy to something in the product? Or they use it in a way that makes them ill? A question like that rarely has a hard and fast answer, and must be based on a good knowledge of nutrition, as well as of the health history of the individual.
Aside from the most important questions of staying within our professional scope of practice, and not wishing to cause harm, but to promote wellness there is one other million dollar question: Will your insurance cover you if you answer this and something unexpected happens?
At his age his hormones are still doing what they’re going to do and he’ll naturally gain weight as he grows. However, instead of steering him towards supplements and liquid calories, steer him toward whole foods. A kid doesn’t need to worry about a six-pack, and at his age he’ll almost never get this kind of growth again.
Have him eating, eating, eating. Not to get sick, but when he’s hungry, eat. A good protein source, a good carbohydrate source, and a good fat source in every meal and he’ll be fine.
If you’re going to steer him towards anything, steer him towards researching everything he’s curious about, gaining weight safely and effectively being one of them.