I agree with many of the posts above.
First off your son should be eating a lot more whole food meals. If he isn't eating right no supplement/weight gainer in the world is going to make up for that. Chances are if he doesn't like Muscle Milk he is a pretty picky eater as well, as Muscle Milk is definitely one of the better tasting shakes.
My suggestion is True Mass by BSN. More of a premium gainer but it does have a great taste and mixes well. I guess though if he doesn't like Muscle Milk maybe he is out of luck as, like everyone else on there said, that is one of the best.
Maybe try a few flavours of Muscle Milk, they have a ton of different ones?