Sorry man, I'm not in the US. I imagine googling for any doctor that has immediate availability, showing the ammonia measurements as well as the signs&symptoms should get you an ammonia scavenger. In the US phenylbutyrate is called "buphenyl", if that helps.
Also, protein restriction, higher carb intake, and citruline/arginine supplementation are low-risk interventions that may be worth investigating.
His prolactin levels are also elevated but I realize that's like an entirely different issue probably.
I see a bunch of telemedicine doctors. But I'm worried about him receiving blood work at home because of covid. I'm in Georgia and it's very easy to receive the vaccination, I said I was a caregiver to my dad. But in Maryland (where he's located) it doesn't look like they're anywhere close to allowing younger people to receive the vaccine.
Edit:. I hope biden is right, may 1st seems awfully close tho. And then I'm not sure how he would go to get the vaccine without becoming sicker
Dysbiosis can be a cause - if he has a healthy friend, fecal transplant would be decent to try. Or something like rifamixin for ammonia generating bacteria. Candida can also release ammonia so an anti-fungal wouldn't be bad to try.