Haha well technically, he won't tell you how much ash you've collected when you talk to him, but he will tell you how much more you need to make the item of your choice. If you're really curious to know exactly how much you have, if you're confident that you have less than 8000 (probably, as it would take HOURS to get this much), you can ask him to make you a Pretty Desk. If you subtract the number he tells you from 8000, then that's how much soot you have. :)