S.,
I had this same problem with my son (now 19), except the little boy "Micah" lived across the street from us. They were both 5. At any time of trouble, Micah would run out the door saying, "I didn't do anything." I sat down with my son and told him that I didn't care what Micah said or did, but that he (my son) would always have to tell the truth. Our bunny in the backyard was released from his cage - "Micah didn't do it." I tried talking to Micah's mom, but she became offended. My son and Micah still remained "friends". As the years passed and more friends entered the scene, it became common with all the other friends at any sign of mishap, "Micah didn't do it." We moved away when they boys were 14, but we still stay in touch. According to the neighbors, Micah got into trouble at school. In court, still maintaining his complete innocence, the Judge requested a hair folicle test and Micah's parents had a fit declaring that his rights be protected.
My point ... teach your son right from wrong. I never told Micah that he couldn't come over or talked bad about him to my son. My son and other friends figured it out by themselves. My son actually learned from Micah the consequences of NOT telling the truth and so I'm glad we had the experience.
Hope this helps.