How long has this been going on?
I know they're out of fashion but perhaps lying on a waterbed would help for when he's lying down? Might be a challenge to find one in use to try out, though.
Else, I suppose you could experiment with muscle massage. Maybe a physiotherapist could suggest stretching routines?
Although, what I am about to propose might be unorthodox just hear me out. My nephew suffers from cerebral palsy disorder. Born with this condition, he has spent most of his life with uncontrollable and recurrent episodes. His family has taken him to almost every Western and Eastern medicine doctor available to them; yet, he was not able to find the seizure relief he desperately needed. His doctor suggested to his parents he try medical marijuana to see if that can reduce the frequency of the seizures. Almost instantly, his seizures went from ten times a day, to only two a week. They got his medicine from a company called Quanta and he only uses it in vape form. I have seen it with my own eyes and it works. Please give it a try because the children deserve it.
There is one answer I can give you from the flakes side. Concept is called "earthing". Replace the bed sheet with a sheet that is basically plugged into the dirt. Sounds rediculous but it does work for me, without the loss of, shall we say focus, that comes with baclafin