Cookie monster have you tried pedisure instead of boost? My son likes it and I think they have powder form that you may be able to add to regular milk. Also, we have been battling my our son’s weight too. Through his pediatrician we added an antihistamine medication called periactin that is used for its side effects one of which is increased appetite. It has helped us tremendously. Another side effect is drowsiness so given at night it can help sleep. It might be worth a discussion with your doctor(s).