There are plenty of things he can learn in Juniors. He's far from a finished product and needs to get stronger, more physical and work on his positioning. He has all the tools but I'm not sure he's quite ready to be an NHL defender yet.
He seems to have plateaued maybe in junior and maybe it's best to get him around professionals and the Bruins coaches? I don't know... Another year wouldn't kill him I'm sure, but i bet the Bruins have him up all year..