I'm currently writing a story (contemporary setting) where the main character suffers of a five-year-old bullet injury. The idea is that the injury causes him to walk with a limp that varies from quite noticeable to almost crippling. The injury was caused by an assault weapon and was badly treated at first, leaving him with the occasional chronic pain despite finally being able to seek proper treatment and physical therapy after two years. By the time the story begins, he's mainly walking with a crutch, but occasionally walks on his own when his leg doesn't feel too bad.
So my question is a simple technical one: which part of the leg would be "best" suited for that kind of injury? It needs to be severe enough to cause all the problems listed above, but not severe enough to deprive him of that leg's usage. Also, how long on average would physical therapy last after such an injury?