In the service of "teach him to walk prior to attempting to teaching him to run", maybe you could start with a discussion of software security, and then show him an example of how bad guys can use good code that looks trivial to do things you don't want them to do. You can come up with numerous vulnerabilities in application code using any reduced subset of Java which he understands. Without that background, he'd have to learn an awful lot of Deep Magick about J2EE to understand what is happening here.
Why not use something nice if you have the chance? Using your arguments about fundamentals, it's not even necessary to treat him computer science, he could just learn maths and he has all the foundations he needs.
Only if all your fundamentals are restricted to what Java implements. A student that knows only Java may have trouble using multiple inheritance, first-class functions or prototype-based inheritance. I've seen a lot of Java programmers who cannot grasp these ideas and get utterly lost. It's not fun to watch.
I'm biased as I already knew Java when I learned Scala, but Scala, if you ignore the crazy parts (e.g. don't look at the scala own sources for example) then it's quite a simple language. Much simpler than Java in my very humble opinion, it feels a lot like Ruby / Python to me actually. Another advantage I find in Scala, is that for new learners, they get the safeguard of a typed language and all the protection that comes with it, if you write incorrect syntax, you get an immediate feedback, and for new learners, I think it's a great feedback loop.
Although I really like Python, I found that implementing things in it for the Udacity CS courses for example, is much harder for me (just because I'm less familiar with Python probably) than Scala, I tried implementing the same homework in Scala, and it was much more intuitive, especially collection handling, and writing "functional style" code, and I didn't have to go to the web and read the docs every seconds because of the notorious, loved to be hated, heavy but yet spoiling comfort of the IDE's content assist.
Anyway, it's a matter of taste, and I like the Scala flavor.
But here I am spending more time on the Ruby tutorial than on the Play framework tutorial, so I guess the language itself is not all that matters...
I don't use Java much these days but I am teaching my kids OOP using Java. I'd like to also take the opportunity to show him where the warts are.