I think you’ve started at a good place with the ESSENTIAL line. Similarly, most of the material released in color in the new EPIC format would probably fit the bill.
I’d recommend any of the Marvel Adventures books. If you want something more fun and silly, go with Mini Marvels by Giarrusso (recently reprinted in an ultimate collection) and, if you can find them at a reasonable price, the Franklin Richards books by Eliopoulos. If you’re wanting books in the main Marvel Universe, I’d look into Runaways and/or Pet Avengers.
Based on your question, I think most books that are not part of the Max line would work. Your local shop should be able to help you find more specific things too.
Finally, I would suggest anything you buy, read first and/or have his parents read it. First, it helps make sure it’s appropriate. Second, if there’s anything questionable in the book, he has people to help put things in perspective. And finally it gives the kid someone to talk to about how awesome the story is.
Just my input of the subject, but it works with my nieces.