When I saw Boas mentioned, I had to look at your post.
Knowledge is the best thing that humans can give each other: it's our best hope for a better future.
He (or you - if you're not familiar with it already) might enjoy Morse and Feshbach. It is a classic (reference) text, and, at nearly 2,000 pages total, there's bound to be something slightly new or slightly forgotten to be seen in the two volumes.
It does sound like he has a really good mathematical foundation from what you've written. Hopefully, more kids are able to get calculus in high school now. (The internet will help with that.... However, my old high school - for whatever reason - still only offers a "topics in pre-calculus" course as its highest math elective.)
I don't remember exactly, but I think that Reif has a discussion or example relating to neutron diffusion near the end of his book, Fundamentals of Statistical and Thermal Physics. I don't have a copy on hand and just don't quite remember. That might be fun to check out.
As before, all the best to your son's future endeavors in the nuclear engineering Ph.D. program. It's cool that you care to be involved.
= new reply since forum marked as read