Is he still in 4th grade? I know more about resources for middle/high schoolers than for elementary schoolers. One recommendation that comes to mind is
The Number Devil: A Mathematical Adventure by Hans Mangus Enzenberger --- this has substantive mathematical content and I've found that it's quite popular among gifted children who encounter it.
Three items which may be useful to you in working with him are
Vision in Elementary Mathematics by W. W. Sawyer
Arithmetic for Parents: A Book for Grownups about Children's Mathematics by Ron Aharoni.
I haven't had a chance to look at the last one but it's recommended by Alexander Givental, a Berkeley math professor and parent who I have very high regard for.
I think that Raymond Smullyan's books of logic puzzles are good for gifted children.
Thanks,
He hasn't reached the 4th grade but I was teaching him above his grade level math.
Most people form their impressions of math from their school mathematics courses. The vast majority of school mathematics courses distort the nature of mathematical practice and so have led to widespread misconceptions about the nature of mathematical practice. There's a long history of high caliber mathematicians finding their experiences with school mathematics alienating or irrelevant. I think this should be better known. Here I've collected some relevant quotes.
I'd like to write some Less Wrong articles diffusing common misconceptions about mathematical practice but am not sure how to frame these hypothetical articles. I'd welcome any suggestions.
Acknowledgment - I obtained some of these quotations from a collection of mathematician quotations compiled by my colleague Laurens Gunnarsen.
In Reflections Around the Ramanujan Centenary Fields Medalist Atle Selberg said:
In his autobiography Ferdinand Eisenstein wrote about how he found his primary school mathematical education tortuous:
There is some overlap between Eisenstein's early school experience and the experience that Fields Medalist William Thurston describes in his essay in Mariana Cook's book Mathematicians: An Outer View of the Inner World:
Thurston's quote points to the personal nature of mathematical practice. This is echoed by Fields Medalist Alain Connes in The Unravelers: Mathematical Snapshots
In Récoltes et Semailles Fields Medalist Alexander Grothendieck describes an experience of the type that Alain Connes mentions: