> Is he supposed to owe the difference in sale price between if he painted it, and if he didn't?
Damages should be limited to the sale price, not fluctuating market value. What if a painting loses its value because the author does something disgraceful - can the buyer sue the painter for damages? Should all painters be careful about their public standing or anything they do that might affect the market valuation of their work, on threat of lawsuit from their buyers?
The buyer should have exercised due diligence and obtained proof of authorship 40 years ago. He only bought the painting itself, with no paperwork. So that's what he has now, just a painting, with no proof. He has exactly what he paid for.