I don't know if this is "Aspergers" symptom or not. I also have an 8 year old son diagnosed with Aspergers and he does nearly the same thing. He doesn't really thrash so hard, but he does sleep with me and tends to kick and grab a lot. It seems he needs to be tethered in order to relax enough to sleep. The kicking and thrashing around is more of an ADHD thing - which you see a lot of in Asperger kids.
My son is terrified of sleeping alone and started sleeping with me as an infant. I have tried many ways to get him into his own bed - but with bad results. I am not willing to lose sleep every night because he will call out and cry relentlessly, or have him work himself into an asthma attack because he's so afraid. It's a problem, and for now he's sleeping with me until I have the time to work on a solution.
If you have the time and energy to do this - I think it may work to have him sleep with you in two twin beds side-by-side... and then gradually, over the course of weeks, move them apart. This is what I plan to do when I have time... this battle is not the top battle on my list of priorities right now.... but I do plan to get to it before he's a teenager
**By the way, Melatonin helps calm the sleep quite a bit!! !!