MRI results back today . . . apparently a pretty rare thing for guys (much less guys his age). Ishiofemoral Impingement Syndrome. Sounds like there's an area where the sicatic nerve passes through that is much smaller on my son than most people. That's causing the problem. My chiro thinks there are some things that can be done to treat it now that it's actually been pinpointed. There are some more invasive things if all else fails, but at least we apparently know the actual cause and can pursue treatment.

We'll see how it goes and may end up following up with some of the recommendations here. Thanks to all for the feedback.