I agree he's too short for a CB.
Nevertheless, I think you're a bit harsh on the guy generally, and it seems to me that you made up your mind on him in his first few months and haven't modified it since. In that first period he struggled with the pace and physicality of the PL, got caught in possession and dispossessed a bit too often - but he has adapted well enough imo.
I haven't any stats to hand, but just from my own observations he seems to win quite a few headers despite being a relative short-arse.
He's no world-beater, and if we can get better we should, but I think he's a solid enough back-up.