What position would Austin play in the NBA?
Depends on how well he can shoot, play off the ball (and how much muscle-mass he can gain).
There are plenty of very successful shooting guards in the league around 6'3'' or 6'4'' (Monta, Eric Gordon, Dwyane Wade, Jason Terry, Marcus Thornton, O.J. Mayo, Ben Gordon etc etc).
If he can't shoot or play off the ball, though, it'll be hard for him to stick at SG. Monta kind of has that problem, but he's been allowed to dominate the ball in GS and he's improved his shot.