Protein is very important to the healing process after bariatric surgery. It is also recommended that you drink protein shakes between your meals. after bariatric surgery liquid protein is the best one.
It is ok to drink protien shakes, just remember he'll have almost no stomach at first so just a view ounces at a time will satisfy his hunger. It takes 6 months to fully recover.
After bariatric surgery, patients require 60-85gms of protien perday. doctors may recommend you to stick to liquid food for some days. So it becomes necessary to drink items that are protien rich.