From your short description, your son sounds very anxious to me.
I would agree with lilomeme - if he's on medication, I would check with your doctor and look up the side effects of the meds to see if that is a possible cause of the increased stimming and aggressive behavior. Stimming (self-stimulation like hand-flapping) is usually a self-calming behavior; the general feeling on these boards is that it serves a function for people on the spectrum and to leave it alone unless it involves self-harm or destructive behavior (and usually in those cases, the recommendation is to redirect to a less destructive stim.)
Otherwise, is there anything else going on in particular that could be causing anxiety? 13 is a tough age, it's possible that it's about puberty - but if there is any upset in schedule or major change in his life, you could be seeing his way of managing it.
At the stickies at the top of this board, there are some posts compiled into an "index." There is information there on aggressive behavior.