Good friend, bad GM/Player, what would you do?

[Man_Over_Game]

So he clearly enjoys it, or a part of it. Have him collaborate rather than control things. Shift his position from a standard player to hybrid side-DM and NPCPC. That way, he plays within the story that he helped create as an NPC that provides some kind of value to the party by just being there.

For example, he could be a ghost of a prince that's guiding the players into solving his murder while stopping the criminal underground that is mostly unrelated to the prince's death (red herrings, and all that). He writes everything the Prince would know, or would have experienced, and offers some minor benefits in combat, and now he can interact with the creative side of things without interrupting people's fun.

Have you do most of the actual DMing while he contributes in the creative sense of things.