Post game as in after the credits? There shouldn't be any reason you can't talk to him, unless you have no mithril ore with you. You can forge 6 different items with mithril (3 with each smith), and there're 6 ores you can find throughout the game (either as quest rewards or hidden in dungeons).
Check if you have any Mysterious Orbs left in your inventory you can turn into mithril ore. If not, then you probably still have a bit of exploring to do before reaching 100%.