I have two little boys the exact same age. My first went through this phase at 6 months and I was horrified. My second, now a year, is in the phase now. I actually think NOT reacting to it is what made it go away the first time around. We try to explain that to my older son, but he's very upset by it and is always yelling back at him. I think if he can learn to ignore it, the baby will stop. It is an annoying habit, but it's one we always look back and laugh about with the older one...it will pass! Good luck!