You can't really level up with him 100% of the time. He will be doing quests, quests and more quests. And also some exploration, the hunting log and the occasional dungeon. But his pace will be slow.

You will be doing your hunting log and roulettes, with the occasional FATE and guildleve. You can join him in the dungeons, and also help him out on the hunting log and exploration parts. Your 50% experience bonus mean you can level much faster though.

The biggest help you can be to him is to guide him along with any questions he may have. Once he gets to 50 is when you can really start playing together for almost everything, barring only the roulette.