I would talk to the teacher about moving his AR level up. Easy to ask them to base it on his aptitude testing rather than grade level.
We have liked the AR testing based on aptitude. It has allowed us to give her more difficult books with assurance that we are not pushing it too far for her to comprehend well.
We usually just buy our own books on Amazon. You can ask the teacher for recomemendations. Our school sent a good reading list by grade level. I just found the right level for our child and was able to get some good suggestions. Fortunately, our kid is getting into YAF (Ender's Game/Hobbit) level that I know a lot better than kids book.
