Whatever you do, just don't do this:
This is the perfect example of how a DMPC can be bad for a game, being the star of the show.
I believe the best course of action depends on how the players feel about him:
If the players like him, either make him leave for some reason that will be a foreshadowing, or make him die in the hands of an important enemy that you want the players to hate. Or he might be kidnapped.
If the players don't like him, simply make sure he dies during some random encounter. Make an enemy party ambush the PCs and concentrate fire on the DMPC first, killing him.