I think telling him is the worst idea. Buy him out and when he gets more stock, buy that. Good find though.
I'd buy out all of his coins, then set a up a booth right next to his with the correct prices listed. Then I'd take a picture of the look on his face when he finds out what they are really worth..... Then I'd take a picture of his face just at the same exact momemt that my karma ran over his dogma.