I don't know of any websites, I'm afraid. You might try The Whistle Shop for books (
http://www.thewhistleshop.com/catalog/tutorials/tutorial.htm).
I learned my first few songs from the book that came with my whistle, but after that basically learned everything by ear - that's the trad music way (and, according to several music teaching methods, a much more important skill for beginning musicians than sightreading - I happen to be a firm believer in this myself). Try challenging your son to figure out how to play some of his favorite songs.
