I'm guessing the father is actually pretty powerful and could become an S rank but while he's still an E rank he met something like absurdly powerful that beat him and ended up cutting his leg, but he didn't realize that thing he fought is actually powerful (beyond S class could handle) and thought
"huh, maybe I'm not as great as I thought. Maybe I'm not even strong. I lost my leg and opportunity to rise rank, guess I'll retires ".
Or something like that...