If he's out for the year and leaves this year, his pro potential is purely as an athlete. If he takes the hypothetical redshirt, comes back and has a successful senior season, he may get his shot as a QB. His decision may come down to how much playing QB at the next level means to him.
I'm still holding out hope that this injury is minor enough for him to be back for Sparty, but I'm not optimistic. Get well soon Brax!