This is pretty straightforward. Matt's quests are mostly instances of other things found in the world. If you've completed them all prior to talking to him, it will be as easy as talking to him a few times. Then he'll have a loyalty mission where you fight zombies and follow Nightblayde around.
Also, you MUST ROMANCE him by pressing X on him in the ship. I forgot this step because it wasn't listed in the description. I couldn't figure out why it wasn't popping for the longest time.