It can be difficult deciphering how children react to different thing and he could be making it up. But if hes complaining during the night, I'd wonder why he would make that part up? Maybe get a cheap memory foam topper for his bed, see if it has an impact on his movement. Try and cut down his fluid intake, just so you can eliminate any possible theories. Do get him to a paedatrician though, as they can give you in depth insight and reassurance
I think you should really be aware of it. Let his pediatrician know. I suffer from back problems and I would hate to see my son suffer from it too. I hope all goes well, may God bless your son always.