Ok ( Using D&D like Stats )
Str and Dex are reduced to 1 or whatever is lowest - He cant move or perform any activitiy but blink breathe and piss in a tube.
Con is reduced in 1/2 affecting his HPs etc
He has no abilities/attacks or physical skills they are effectively 0.
He has no attacks unless psychic or someone throws/drops him and his wheelchair at the party.
His charisma could be intact though it could affect how folks see / interct with him so he could take a hit here as well.
His mind is his weapon now, and his charm/wit.
He could be very loved and respected by his allies who will rally to his side and seek to destroy the party. I see him waging a shadow war against the party, killing off/defaming/hurting friends, relatives, allies, engaging in smear tactics or trying to set them up for a crime, lose face, seem like villans. He would try to destory their credit <bank>. He would hire assassins to kill/maim them.