Sorcha - having made a few instruments myself, but seeing the flaws I made in them, I don't think they are worth anything even tho other people praise them. But because I don't value them, I wouldn't sell them. But I have given them to people who appreciated them.

I would suggest that instead of trying to convince him of the violin's value (your opinion vs his, you're not likely to change it), rather try to convey how much you appreciate/desire that particular violin (your opinion vs something he has no opinion on. You're much more likely to persuade him). That takes the question of quality out of the deal, which seems to be where the block is.

If it's money itself he is resistant to, propose a barter or something. If it's the selling of the instrument (sometimes it is hard to part with a "baby") he's avoiding, ask if you can rent it from him. Then you'll get to play it, you can pay him for the quality, but he doesn't have to separate with it right away (even tho you'll be the one in possession of it). At some point down the road, when he's gotten used to you having it, suggest rent-to-own, or buying it outright for what you've given him in rental fees.

I hope some of this has given you an idea on how you might get the thing.