can’t get him back. there is a forced fight with him.

however you can take his heart (item drop) to a cleric once you get back to the main land and they’ll cure the geas placed upon it, ensuring he goes to his proper afterlife I believe. (i haven’t done that quest in 15 years or so)

I mean, he was a thief class. He had it coming

You know you can wait your sentence out and get your stuff back. You only lose things that were marked ‘stolen’ in your inventory.