Try asking the race track folks, ask around, check at the bigger tack stores. If there’s racing, there’s sensibly priced lay up facilities that can also handle a simple quarantine without having to price care to cover a fully staffed on site clinic with surgical suite. They don’t advertise though.
Doesn’t sound like he’s “been sick for a very long time” or somebody would have picked up on that and he’d look like it…he may have been an asymptomatic carrier for a long time though.
Might pay to get at least a consult with another vet with more of a background in current protocols, not every vet can be an up to date expert on everything in today’s rapidly evolving veterinary environment. There may be more you can do.