dear docsmom,
Without being able to help you with the diagnostic process I can give you a suggestion.
You could either try massaging his lower back or his feet.
Both are easy to do. You take some oil and rub in the direction that your son likes (check this beforehand)
and you massage/strook repeatedly until you either see the area turn a little red or feel it become nice and warm.
If he doesn't like it... do not do it.
If his feet do not become warm, tend to become white, stop... that is a sign that his body does not find this a good solution.
I wish you both well.
Esther