My girls LOVE it. Start with just a coating of his bowl because you don't want to shock his system. But he'll tummy will get used to it and you can up the amount quickly. It's got eggs, which is good for him and calories for the weight gain.
I also gave my older girl Onyx when she was 15 Ensure for weight gain when she would decide she wasn't hungry. Just to keep weight up. But I think they like the eggnog better.