I think you just answered your own question. If he's reading Magic Treehouse, enjoys it and can understand it, I would aim for books at a similar level or slightly higher. The Magic Treehouse books seem to be at DRA 20-30 or around M depending on which guided reading scale you use. Have you tried the non-fiction companions to the Magic Treehouse books? They are often more advanced than the actual stories and contain a lot of new vocabulary.
This is a good list:
https://www.goodreads.com/shelf/show/level-mMy child, who also liked Magic Treehouse books, really enjoyed Freckle Juice and My Father's Dragon.