He can play together with you at any time. As Atoli mentioned, that's exactly what the level sync function exists for, to ensure that players of different levels can play together on a (relatively) equal basis. So there's no reason at all for him to rush through the actual game. In fact, playing together with him as he explores it is a fantastic way for you to spice up your own game with the variety of content he'll be reaching.

Don't worry, it goes fast. He'll be stuck at the end with you endlessly repeating the same thing over and over again soon enough.