I have had students who have had this operation and also a nephew. They are all so much happier afterwards.
The thing is that if your son can only breathe through his mouth then he can't smell things, if he can't smell things then his sense of taste is, at best, muted.
We are designed to breathe through our nose and there is a much healthier air cleaning process from nose to lungs than from mouth to lungs.
I have also heard reports from adults who have had the op that their energy levels doubled because they slept better and their mouth didn't dry out at night (which is really bad for your teeth).
Operations are very scary things and I hope that your son reacts to the treatment he is currently on and all goes well.
But if not take hope in the fact that it is a very routine operation and that your son will have a whole new lease on life afterwards. He wont know himself for all the new smells and tastes.
Best of luck , keep your chin up.