If you fire him, he will simply go to another department and hire on as a lieutenant.
If you keep him on at a probationary patrolman’s position or equivalent, that will be his final rank if he quits, which makes hiring on at a higher rank problematic. His retirement pay and severance pay, if any, will be calculated based on that rank as well. And then all the people he bossed around will now be ordering him around.
Well, you can’t put him on the street, because his fellow officers will never come to his aid if he is in danger...