Looking at where he is now and looking at his dad I'd say you're pretty much on the money where he'll be in 3/4/5 years. He's going to be a load. He's also stronger than he looks which is saying something. He manhandles older guys his own size. That empty net goal against Canada in the World Championships--he just knocked Shea Theodore right out of the way. That's a real deal NHL defenseman--that's impressive.
He's going to be a different kind of player than Hughes. We'll see who turns out better.
Can't really do that though, very possible that he is more like his mom.... Though she also looked to be 5'10" or 5'11".
I don't really care if he grows to 6'4" or how much weight he puts on (he is going to put on some whether he wants to or not) but the narrative perpetuated by the media and Devils fans that he's a man already and close to physically mature is a false one. You can tell he hasn't spent a ton of time training off ice (I can all but guarantee that Hughes has put in more playing with the NTDP) so there is a ton of room for him to grow in terms of power and explosiveness.