I know! He can fall into the sea on the way to Besaid from an attack from Sin or a Sin spawn. Then, if he returns, he could just say he washed up on a shore somewhere and made his way back to the mainland to find his fellow guardians and summoner.

It's stupid, but kind of easy to do since we'll be going on a boat anyway. Until then, he'll just have no say in any matter and act as a side character that doesn't need controling.