Thanks for a prompt response.  We have tried risperdal in the past.  It was discontinued due to concerns of weight gain and family history of diabetes.  It worked well in the beginning, but I suppose he became tolerant.  That is when we switched to adding stimulants and eventually moved to other atypical anti-psychotics like Zyprexa.  We've even wondered if he needs an anti-depressant.  Perhaps the tantrums and aggression are part of depression.  We are in ongoing discussions with his treatment team.  He is currently at a residential treatment facility in Chicago, IL.  I will let you know what happens with us.  Again, thanks so much for your suggestion.