That... could work. Most of his combat potential isn't dependent on his fitness. Perhaps start off with an arc where he starts to show potential as a hunter and possibly reconcile with his family. Then BAM, suddenly an injury paralyses him from the waist down or he loses a leg or something. I prefer dismemberment but I'll see what happens IC. This is actually a situation his family's okay with because scars earned in battle are proud ones etc. They suddenly switch to being warm and coddling albeit still with a condescending overtone.

After a heart to heart talk with his team or something he comes to realise he can still do a lot of good on the front lines because hey with his semblance the footpedals are redundant (as are pretty much most manual controls, he originally designed [i]Unsubtle[/i] for people without a semblance). That means he has the world's most lethal wheelchair and [u]why not[/u] put that to good use?

So he gives his family the proverbial finger and gets back in the saddle. Preferably just in time to play cavalry, let Aurellius surf [i]Unsubtle[/i] into battle for a glorious Big Damn Heroes moment.