From process of elimination, I'm doubtful that it is indeed arthritis, but Pentosan does no harm and can be used as a preventative to ensure joint health for the future, so there's no harm in trying it anyway. If it IS muscular, which I am leaning towards at the moment based entirely on symptoms, a good chiro/acupuncturist will sort it out. If it's something to do with the way his hooves are balanced (possible, I've been trimming them and I'm not the greatest trimmer around) then the bodyworker we use will be able to diagnose and solve the problem that way as well, as she is a fully trained and qualified barefoot farrier.
And then if it reoccurs with no obvious trigger and does not disappear in the course of a few days I will look into more in-depth diagnostics.
We only just got a specialist horse vet in town, and she's so new to town that I don't know if I can trust her judgement or not... the head vet of the practice seems to like injecting the horse with expensive drugs if he's not sure what's going on, so I don't trust him either.
Now you see why I'd rather sort this out without a vet if I can?
EDIT: and I totally forgot to mention our EBW is also a fully qualified vet, so if there's something she sees that is a medical thing that needs a vet, she'll say something. Her interest is in the horse, not getting paid at the end of the day (though of course she charges like a wounded bull, because she is awesome and it's a way of making sure she can keep up with the demand)