subdude,
My first (and currently only) rifle is a "black rifle." I'm a little older than your son (31). For me, it was a project, something to tinker with, that has really been opening me up to the shooting world. My initial interest did spring from video game and movie iconography, but the lasting thing that has really fixated me on the platform is the ease and accessibility of working on it. Currently, I'm buying a part here or there every payday to put together a 6.8mm upper and to start on an AR-10. In my mind, there is a special kind of accomplishment that comes with taking a pile of metal parts in front you and assembling them into a functional, accurate, and incredibly fun to run weapon system.
Although I'm not military/LE, I believe there's legitimate fun to be had in watching Magpul dvds and practicing reloads, etc, then hitting the range to squander $50 in a minute and half with a system that you put together. Personally, I am eagerly looking forward to attending my first 3 gun match some time this summer- a chance to find some of my weaknesses, but also to run a course with a rifle I built. Not sure if your son is interested in competing or building his own AR, but I believe this kind of adaptability and personalization is pretty appealing to a lot of people, when it comes down to it. Maybe you and your son could get the parts and build one together? It's a project I would love to do with my own father at some point. Best of luck!