If he’s not lifting weights yet, I wouldn’t expect much muscle gain at this stage. Which is OK, he’s a kid. If he’s a string bean and you’re just trying to get some mass on him, then I’d look at total caloric intake… find ways to get more carbs & fats in there too. He’s young enough he can get away with eating “crap” food, as long as it doesn’t turn into a problem where that’s ALL he wants to eat.
Casein thickens up more than whey protein, so he’ll want to add more water or just eat it like a pudding… the chocolate casein is great as a pudding, mix with some PB in there… yum.