Im wondering if that guy didn't get hit by a vehicle with the know on his front leg? Also looks like winters can be a factor in your area. That buck has a lot working against him. Especially if he has two bad legs. He's still gonna rut as hard as he can. Add a nasty winter and I don't think he'll live.
I felt the same way as u about a three legged doe I was seeing all the time on my place. I let her live 2 hunting seasons. Than we had that brutal winter 3 years ago, and I found her dead.
On the other hand, years back I did a full body mount on a 3 legged buck, for a guy. I wanna say that buck was around 150. The buck lost his leg young, and the hunter had 2 or 3 years worth of pics. That buck kept getting bigger every year. That was during a stretch when our winters were tamer, though.
If u let him live, his survival will come down to winter severity-imo. He looks in the 4 year range to me.