Maybe you go get a second opinion. That does not sound right to me. For the doctor to tell you that he's fine when he is obviously is not is kinda weird.Good luck.
Does he see his GI doc or the surgeon about this?
Gall bladder surgery can cause a lot of different problems as the body adjusts to the bladder being gone.
He could have bile in his stomach which will make him nauseated and can make him throw up (it's green or yellow). Certain foods can make that worse. He should try to restrict fats from his diet for a bit and that may help.
However, with my own experience, I would suggest he pressures the docs for answers (go with him and back him up) and if no explanations are given, switch docs. After 8 years and 3 docs I have found that my upper abdominal pain, nausea and "reflux" are a result of my surgery. Now that I have restricted animal fats in my diet I am doing a lot better!