If you feel threatened at work but can't quit, what would you do?

That info gives it some useful context. Criminals are often not the sharpest tools in shed re social cues but beyond that if they are career criminals with a propensity for violence they are socially trained in prison to carry around an air of intimidation or potential threat as their default vibe. If that was the environment he was in for many years re-wring the way he carries himself might not be all that easy.

If he’s just an asshole and enjoys making people squirm by being up in their space and is well aware of what he’s doing that’s another matter. There are cops and prison guards on this board who could speak to that potential inclination with much more authority than I could.

It sounds like him being aware personal space is the issue. If I had someone doing this I would adopt a helpful instructional tone. It would be best applied right after he starts getting too close. Adopt a big sister or Mom tone.

“Look Bob, just as some news you can use, in conversations where we are discussing instructions or information you’re got to stop looming. You’re a big guy and people need a certain amount of personal space to be comfortable. If you’re going to have professional conversations with people you need to give them this much distance (show him how much) between you otherwise it’s awkward.” Ok? Great!"

Go back to your discussion.

This way you are doing him a favor and he will probably be grateful for the social cue. If he persists with looming it’s deliberate and you need to take it up a notch with your manager.

I’ve worked in several jobs with high risk clients, including a jail, an attorney’s office and now in home health. The main thing I know is you must assert your authority, kindly and politely but assert it you must do. It’s not like normal male/female relations. You are the staffer, he is the client. If your manager does not have your back on this, go up the chain of command.