All good advice here...
World bank notes are a great way to go, and of course you can get plenty of 1970's $1/2/5's for little over face. If you go to a show make sure to go to the KID's sections, or ask dealer's for those notes (bargain deals). Check the UPCOMING EVENTS thread on this site for the shows nearest you.
Also, consider buying a USED Charlton catalogue at first, to see if your grandson takes to it or not. Plenty of 2007 19th Ed will be available at a fraction of the next issue's cost. The book (bible in paper money) is well worth the price-- if not for the pictures/info alone. While the 2007 Ed prices may be somewhat dated and newer prefixes unlisted, it is likely your grandson will be more interested in the lower denominations of pre-86 (and there won't be too huge a $ difference with most common notes). Of course, most people on this site will advise you to stick to higher grade notes (this IS good advice). You also should haggle a little over the prices listed at shows (most dealers expect it!) especially on more common notes.
It would be worthwhile to point him in the direction of lower denominations. I've seen member's kids get very interested in collecting 1954 Devil's faces...which are collected all over the world (one reason for their popularity).
If you take him to the odd show (which can be fun/interesting for both of you) you can browse the various tables and see what piques his interest. You can also pick up the note holder sheets, and other supplies. Its a great place to meet other collectors...and there's plenty of young and old collectors alike. You can learn a lot there too.
While its true that Canadian bank notes leave most coins in the dust as a potential investment -- most kids start out collecting just for the eye appeal and could care less about making money on the hobby. At the show you may discover that he truly likes coins instead. I would guide him in whatever direction he chooses since it will be his hobby (not yours) to embrace. Hey... and then you can collect a few notes for yourself!