If his character portrait vanishes means that he has been removed from your party. Imprisonment and Flesh to stone are some of the ways it can happen. If his body is not destroyed cast Stone to Flesh and talk to him.

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You cannot escape the inevitable.

If You still have his icon then : 1. press Ctrl+Space 2. press CLUAConsole:EnableCheatKeys() 3. press Ctrl+R While the cursor is on his portrait

U really can't resurrect him if he is smashed to a thousand pieces when he is in stone state.
Best idea is to load a previous games.
I save around 6 games just in case something goes wrong, cause you never know...
Yeah, planar sphere was a tough quest, very tough, summon loads of minions to do your work.