Yep it is hard to find stuff that interests them and is appropriate for them but stll extends their reading level. In my experience any reading is good reading so yep if he wants to read picture books, go for it. there are some great picture books for older readers like Mao's last dancer, and books by Gary Crew - check these though as some are a bit dark and scary. Colin Thompson (I think) also does some good picture books for older readers.

Also have a look at the magazine section if he is interested in science (scientrific is a good one) and there are others as well that might pique his interest.

Non- fiction are great for building vocab if you can find topics he is interested in.

To extend his reading ability as well talk to him about the books and read them together out aloud so he builds his comprehension and also verbal reading skill, using tone, pitch and punctuation.

I think it is fabulous that you are involved in his reading!!! Fantastic work