So I asked Spouse Weasel about this, since he’s a clinical psychologist who specializes in OCD and tic disorders (including Tourette’s.) He says it depends on the part of the brain that’s injured but it’s certainly plausible. If we frame aggression and impulse control issues on a spectrum, swearing would be something on the low end. So it’s possible this guy has a problem with low-level impulse control. Coprolalia shows up for roughly 10% of people with Tourette’s, and the tic doesn’t necessarily have to be randomly shouting swear words, it could also include inserting swear words into sentences that don’t need it. Obviously the usual disclaimers apply - he would need to be evaluated by an expert.
/Spouse Weasel
But here’s the key point for the OP - it sounds like he already has been evaluated, and has the documentation to prove it, and legally I don’t think you’re entitled to know any more than that. This is HR’s domain. If you think he can’t perform the essential functions of his job, I’d start documenting why that is - is he getting complaints from customers? Then you would be tasked with finding equivalent work for him. I don’t know the intricacies of ADA law, I just know when I looked into it myself, it said no other employees (including supervisors) are entitled to know what your disability is, as long as you’ve documented it medically, your ass is covered.