It doesn't matter whether or not DS1's muscular/skeletal issues have anything to do with the fact that he was allowed to be nearly completely sedentary for a few years while I was having surgery after surgery followed by cancer, more surgery, and radiation treatments. And more surgery.
It's entirely likely that some of what's going may be a neurological problem, since his core strength never seems to improve.
It's also true that his problems are not debilitating, and most likely are non-progressive since we've actually seen improvements over the past few years.
Having said all that, after spending the morning at the orthopedist and then a big chunk of the afternoon scheduling with the neurologist and the physical therapist, I can't get past the feeling that none of this would be happening now if I had just been paying more attention earlier.