D.P.
I think he IS in pain - real pain - at least once a night, and that he might be waking up other times *anticipating* pain, and he's really worried about it. Have you tried a placebo? Say, orange juice thickened with corn starch (or whatever juice he likes). Put it in an empty Tylenol (or other medicine bottle) and dispense as usual. If he noticed the different flavor, just tell him you bought a different brand. Try giving it to him before bedtime, saying it should keep his legs feeling great all night until morning.
Try it for a few nights, see what happens. If he's still up multiple times, then try real Tylenol before bed (and then a second dose in the middle of the night, as needed). Poor guy. Poor mom and dad, too. I hope you find relief and answers soon.