If your not finacing it I would assume you would need to secure a notorised sale transaction before paying his loan off. That way he cant wiggle out of it when the title shows up by claiming you didnt buy it.
I do belive if you finance it your bank will make the payment directly to his bank but not sure on the specifics of the title transfer.
Best bet is probably to contact his bank and ask what needs to be done.
here is the contact for freedomroadfinacial I think they are the most used by triumph dealers at least here in the US