Don't know how to find a math mentor, but my son (now 10) played with these kinds of problems when he was younger:
https://www.jrmf.org/activities/Not that he's too advanced for them now! I think that would be impossible. He just is into other things at the moment. You should do them together if it's a new kind of thing for him because it's easy to get frustrated and give up. Plus there's no feedback without someone to bounce ideas off of.
I hope you can find a good math circle or something. But in the meantime, some cool books:
"Which One Doesn't Belong: a Shapes Book" (it looks like a book for little kids, but it's really fun)
"Patterns of the Universe: a coloring adventure in math and beauty"
Martin Gardner's "Colossal Book of Mathematics" (this is really for adults, but there are probably some chapters he would like)
"Things to Make and Do in the Fourth Dimension"
"Beyond Infinity"
"Math with Bad Drawings" (my son's current favorite)
