What is mostly holding the dude back is his lack of battle sense and experience. He has plenty of power already. But as we saw, he spent most of the fight so far standing still and being overwhelmed. Or something. But then again, you can't gain experience suddenly out of nowhere, and changing his attitude isn't done in minutes. So, raising his raw power might be the only thing he can do. Still, it's unfortunate that he's mostly concerned, in his day to day life, about satisfying the gluttony through the easiest way, by slaying mobs barely sufficient for the purpose, so he's not exactly getting stronger, apart from the raw values. Your raw values can be really high, though, but you can still get easily slain by someone knowing what they are doing. Someone should tell all this to the dude as he's obviously not realising it himself. He has been travelling now for a while with Myne, yet he never asks her to fight practice matches with him. The old knight as well had to beg him to do it.
I suppose it's the bulk of Fate's life as a beggar and bullied person that caused this. Too little and too poor food impaired his brain development, and being a doormat for the arrogant skill-blessed people made him a person who won't expect anything from others, least of all ask for anything. He's really passive too. I can't say following Roxy, in hope of helping her, is a sign of any particular initiative. It's just a sign he wishes to keep living under her supervision, so she has to be alive for that to happen.