I'm so sorry to hear how traumatic this event has been to you and your son. And I agree with Kristina – stick directly and faithfully to facts, and keep your emotions out of it. You will appear stronger, more reasonable, and in better control. And I can guarantee you that the other boy's parents are probably just as protective and emotional as you feel. Emotions are important, but don't usually help progress toward a fair resolution.
You don't say much about why your son was blamed, other than he picked up the other boy's shoe. You say, "my son is a very playful kid as any child would be," and statements like that are sometimes used by parents in defense of serious misdemeanors in their own children. I'm sorry, but it doesn't sound like we have heard the whole story. Does your son have previous behavioral issues at this school? Did he do anything with the shoe besides pick it up – perhaps try to keep it away from the other child? Does he ever tease or pick on the other child? Is he ever part of a group that does?
If something like that happened, I can see that it might have sent a child with poor impulse control into a state of frustrated rage. That doesn't make the attack okay, but it does help explain it. And if the other child is "disturbed," that generally means consideration must be made for his reduced capacity for self-control. Whether or not he should be mainstreamed with "normal" students is a separate issue that the school may need to examine. Those decisions may be out of their hands, however.
Again, I'm sad to hear how challenging this situation has been. I would be in anguish if my grandson received a beating from another child, for any reason at all. I hope all parties are open to learning from it so it won't get repeated.