At 11 years of age, unless his diet (daily food intake), is very low in protein.
I would advise against giving a child protein powder.

They give protein bars to starving kids in Africa who have very little food

While protein is not steroids and is very safe, what is the point in giving any one extra protein if they do not need it.
He will more likely be taking in way more calories than he need, which will make him gain weight including fat.
Save your money.

Also there is no magic pill or shortcuts in training, giving youngsters the impression that they need extra supplements such as protein or they will not make any progress is absurd.

Tell him he does not need anything, the strength gains from 11-21, are phenomenal.

Developing the right mental attitude is more important than relying on any pill or magic formula.

Be careful which protein powder you give him, if any because alot of protein powders contain creatine.
Again a safe and effective training supplement but not aimed at children.

Go old school, and a pint of milk everyday will be enough protein he needs.