He might be former military or law enforcement. Certain branches/designations of both (although we're talking dark, hardcore shit) will subject you to controlled chemical munitions exposure to build your tolerance to it.
In jobs like that, you're certain to run into exposure to it, & whether or not you can keep your shit together could be your line of defense between life/death.
It's not uncommon to find these guys (a few women, too) all over the place in the service/bar industry as bouncers after they retire, leave, etc. Most of them won't say much about it, if anything. But they're out there.
So yeah, Chad Thundercockbouncer might have been a fuckin Navy Seal in the aughties, or something. Just a possibility.
E: She-quality.