Hello and thank you for writing to JustAsk!
It is not unusual for sibling rivalry to happen even when there are gaps in ages. Much of the time it is considered a normal developmental stage. However, it is not common for the type of behavior that you are describing. Therefore, I would suggest that you may wish to discuss these behaviors with a professional in your area, such as a mental health or a school counselor. Also, it would be important to journal if this is an isolated event due to low impulse control (part of what we call executive functioning) or part of a host of changes in your son's behaviors.
In addition to information found on this website please feel free to check out the following:
From KidsHealth.org
http://kidshealth.org/parent/positive/family/sibling_rivalry.html
SIBLING RIVALRY a book by Adele Faber (easily found)
The BIG Book of Parenting Solutions by Michele Borba www,MicheleBorba.com
KIDLUTIONS -offers downloads of materials for FREE regarding Anger Management
I hope these resources are helpful to you.
Louise Sattler, Psychologist