I would not buy any junk at a "health food store" because the clerks don't necessarily know what they are doing, and they know nothing about the manufacture of the many products the store is selling. They sell from various competitors and are more likely to recommend whatever they get the largest profit margin on. Even if they don't, be aware that most products designed to do one thing (e.g. provide protein) do only that.
It sounds to me like your son has several issues going on, which is why a single-function product may not be enough. I think you're on the right track, mind you, but I think there are several factors. He feels sick in the morning, which could be due to a lactose issue ("no milk for me, Mom"), a broader digestive issue, or even allergies (a lot of post-nasal drip during the night can really mess up the digestive system and change the taste sensations, which might be why nothing appeals to him). The next issue is he runs out of gas, between the late lunch and the band practice. He's underweight. That probably makes him crazy if he's a normal teenage boy. Although uyou didn't say so, I can't imagine his focus is very good if he's really undernourished during the day. If he's going to sustain himself in college with a much heavier workload and no mom to try to shove food in his face in a variety of recipes, he's going to have a tougher time when he goes away.
My son had some issues in high school as well as a serious illness sapping his energy, and he did great with a comprehensive formula that he could take with him and just add water when he wanted. It's not thick like milk products so it was very easy to get down. You're on the right track as well with the idea of a powder that you mix in liquid - premixed proteins break down in about 20 minutes once they're in liquid form. Premixed things often contain emulsifiers that are hard to digest.
But active adults and teens need more than just protein - they need vitamins and minerals and trace elements, and all those things should be scientifically blended to work together in the right proportions. And they need to be highly absorbable. So taking protein without the other ingredients means it's not metabolized effectively and the cells can't utilize it, so the unused portion is eliminated. (Great, you paid for it and he ate it, but the body didn't use it, you know?) And pills are not absorbed more than 30% tops (often less), so they are not effective and can also be very tough on the digestive system.
My son did great in college and built up muscle, had a strenuous athletic career, and continues to get a good amount of nourishment from comprehensive supplementation. We had a lot of help from sports professionals (free from the company) and I'd be happy to speak with you and have them (and/or my son) talk to you and your son if he's open.
This is fixable and I'd be happy to help.