Ok.. did a little research on supplements for teens and found that a normal protein intake should be 1-2g per pound of body weight so if he's 100lbs, 100-200g of protein is needed daily.
If he is not getting that much through diet alone, then using a supp wouldn't hurt, but i would limit it to one a day.
I did go to the Slim-fast website and dug around for some info. They have five different types of RTDs and only two would be good for TJ.
1. Low-Carb Shake: Calories - 190, fat calories- 80 Total carbs - 6g Protein- 20g
2. High Protein Shake: Calories - 190, fat calories- 45 Total carbs - 24g Protein- 15g
As long as the rest of his diet is balanced, he is getting the right multivitamins & minerals, and he's working out smart; then him seeing results won't be too far off.
This shouldn't be an issue though... he has you and you're a smart cookie!!!