Hello..to get through to your son, you might want to do a little role playing. You be the boy your son picks on and have your son bully you. Overreact while playing the other boy so that your son sees how he has hurt your feelings greatly. Then if your son is willing, you play your son and have your son act like the boy he bullies. This will send a message to your son that we all have feelings and can be hurt by the actions or words of others.
Additionally, here are some child friendly books about bullying that may interest your son:
The Berenstain Bears and the Bully
by: Stan Berensatin
Help Me Be Good: Bullying
by: Joy Berry
Help Me Be Good: Being Mean
The Bubble Gum Kid
by: Stu Smith
My Mouth is a Volcano
by: Julia Cook
Your son is only bullying because it gives him a sense of power that he probably does not feel much as the youngest of your children.
Hope this has helped. If you need more advice, just ask.