Behavior issues

Where do I begin? I have been a step-mom for 5 years. My 10 year old son was diagnosed with Williams Syndrome at 5 days old. He has gotten through all the physical hurdles so far. However, his behavior has gotten increasingly worse. He has been on medication for ADD for about 2 years. After many adjustments we finally found one that seemed to be working. However, his behavior is out of control again. My husband and I also have a 3 year old and 1 year old. They are “typical” children. My stepson is so mean to them. He is always hitting or kicking them and we are increasingly frustrated. We are hoping that others out there have gone through the same thing and can give us any info on what worked for them. G wants to be a good boy. He just cannot seem to control himself. We love him but are constantly putting in time out for hitting. He kicks and hits even if we are in the room. He functions at about a 3 year old level but typical negative consequences do not work for him. We have tried different reward programs and nothing seems to be particularly effective. Please help!!!