Behavior in children with developmental delays

My son is almost 4 years old.  He was an extreme low birth weight baby (1.5 pounds).  He has been diagnosed with speech, auditory and vestibular delays.  In addition to this he also has a sleeping disorder which is apparently connected to an immature central nervous system.  He is medicated enough to get sleep, but we have found his behavior is not improving much.

He needs constant activity, he's always 'on' and although we have stopped much of the risk taking behaviors, he will (apparently) defy us and no discipline seems to work.  He might get upset that we're upset, but he doesn't care if we take toys away, give him time out, ground him to bed.  He just screams and screams until he throws up or hyperventilates.

We don't know if his behavior is related to his diagnoses, or if it's disciplinary.  We're guessing it's a health issue and we don't know what to do.  What can I do to help and better understand my son?