If I were asked the question, what do I do if my child's coach wants to switch them from a one to a two-handed backhand.
If your son has already developed a reasonably functional one-hand backhand then I would question why the change. It's probably easier for this coach which may be the reason.
As for one being totally superior over the other, I would point to the success of one-handed players even now in a game dominated by two-handers. Of course, there are now many top players with a two-handed backhand such as Rafa and Novak but the GOAT herd includes Federer and Laver, both one-handers.
You could also make an argument for changing him from being a right-handed player to a left-handed one since Tony did it with Rafa and the lefties have advantages on the court. But most of us would see that as going too far.
As for limiting his future on the court, I might argue that the mere odds and commitment requirements of becoming a world-class or even a 5.0+ player limit anyone more than any method they use.
If he plays competitive tennis and becomes a social player later in life a one-handed backhand will let him last longer into senior citizen play since slice combined with extra reach will carry the day.
