Interrupting

My youngest child, 8 year old son, constantly interrupts people when they are talking. This has been going on for many years. We've taught him to put his hand on our arm (ex. when we are visiting after church service) to let us know when he would like to speak with us. This works most of the time, but he will interrupt other adults talking so he can say hello to his favorite people at church (he's very social and outgoing, lol). At the dinner table at home, he interrupts while others are talking, sometimes by talking, other times by just making noise. We ask him calmly not to interrupt, but it seems to go in one ear and out the other. What training technique would you recommend to stop his constant interruptions? (P.S. I'm on module 1, lesson 5)