It was nice to get him some experience in the NHL, but I say let him play two more games with Nolan as coach, if they think they can work with him then keep him up, but he has a very good opportunity in London this year, so the most logical thing to do is send him down, at least IMO.
I think he'd be good enough to play in the NHL this year, but there's still some little things he can improve on that would make him a much better player with a legit shot at making the team next season. Giving him an NHL stint probably gave him some confidence as he did not look that out of place, but he and the coaching staff probably realized the things he can work on in junior playing for a very good Knights team, guaranteed a Memorial Cup spot, and he's also probably guaranteed a spot on Russia's blueline at the World Juniors (something I'd imagine the Sabres would send him down for regardless). In the draft he was viewed as a project, and he's been surprisingly solid, but there's no need to rush him at all.