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 Unsubtle for people without a semblance). That means he has the world's most lethal wheelchair and why not 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 Unsubtle into battle for a glorious Big Damn Heroes moment.