We are sorry you are struggling with this, there are no easy answers here.  We are glad you held him back if he was not ready, there is nothing wrong with holding back for a year if it benefits the child.  

Different interventions will work with different children.  We actually think that a great beneficial first step that can help you decide how to proceed with your son will to be to get him an assessment from a child psychologist.  Inform the counselor of your concerns, let them speak to your son, and see if they have any recommendations.  They deal with problems such as these all of the time and after meeting with your son, they will have more insight as to what interventions and behavior modification methods would be most effective.  

In the meantime, get him involved in smaller play groups, help him identify his emotions as he experiences them (such as anger), and then teach him healthier ways to respond when he feels that anger.  Also look into educational books that you can read and work through with him so that he can have that perspective as well.  

Please take care and remember that you can call the Boys Town National Hotline 24 hours a day at the number below, free of charge to discuss any parenting issue.  We are here to help, you are not alone!

Counselor, NZ

Boys Town National Hotline-A Resource for Parents and Teens
www.parenting.org
1-800-448-3000