He had better consult with his surgeon. I have had many of those things and I am a bypasser with a stricture, food can't pass through. This is usually a gastric bypass problem but not all VSG are exempt. And if one variety doesn't work, tastes so bad you can't stand it, there are many others to chose from. The kind that works for me isn't one you commonly hear of. After 10 months of protein shakes one does become a connoisseur, I finally found one that doesn't choke me up, has flavors I can stand and doesn't totally break the bank. And lastly your son may have developed a temporary allergy to some of the milk proteins, there are soy-based, or plant- based, personally I don't like them as well but they are a total God-Send for many. There are always answers to every question, just not always easy to find.