Maybe your GI thought that your son would use regular milk to mix it in, and that was objectionable?
Like texasmax said, check the ingredients. Alot of the whey proteins are loaded with junk and sugar.
I've been doing my whey protein shakes twice a day - with almond milk - and it's been great. I like the Dr.Mercola's Pro-Optimal Whey Protein because his is all natural, no junk in it, comes flavored - chocolate is delicious - and comes with digestive enzymes in it already - which makes it so easy on the stomach. Also his comes from grass fed, no antibiotics or hormone fed cows.
Here again, as always, be your own best doctor.