Rebecca -- I regularly make books in the .IMP format. It's not difficult at all if you start with a decent HTML file or a book in Mobipocket format.
Nick Rapallo is a frequent contributor to MobileRead and I use his
programs to create books for the eBookwise. If you're a Windows user, he supplies a very easy to use graphical front-end to his tools.
I don't have an eBookwise right now, but it is indeed a very sturdy reader. It will need recharging more frequently than a reader which has an e-Ink screen, but then again it costs half or even less than half of the majority of other readers. And, for an 11-year-old, it may be a good thing that it won't hold too many books at one time. If you're putting books on it which he will read for his school assignments, then you want to make it easy for him to concentrate on just the book he's been assigned (or one which he has picked for unstructured reading).
There are a *TON* of books for boys available right here at MobileRead already formatted for the eBookwise. And, you can get books from Baen, 110 of them free of charge. Not all of the Baen books will be suitable for your 11-year-old, but he's going to grow up and if he enjoys SF and Fantasy, he might be going back to Baen again and again for more of their free books.