Also feel that he stands far too square on to the winger, which doesn't allow him any scope to direct the wingers direction. That often allows himself to get turned in all sort of directions, to the point he nearly falls over. Perhaps he hasn't learnt to trust team mates to deal with the winger cutting back inside.
He needs to know his opponent. Is he left footed and therefore looking to get to the byline 9 times out of 10? If so, show him inside with your positioning and stance. Etc.
I dunno, I feel like he's not being coached enough on his defensive game.