Could he be feeling anything that is making him "afraid" to walk? Anything besides the pain? I know my DD10 (who also has POTS/dysautonmia)can have a racing heart, burning leg pain, and feel "off-balance" and "heavy" which makes her not want to move about much sometimes...
I also would look much more in to it before you think conversion disorder. PANDAS/PANS is so crazy weird and presents differently.