Hello Valerie and Ronnie,
That is interesting. To be 100% sure if what is going on, I would need to observe and see first hand. However, I will tell you what I suspect is going on, it is most likely one of 2 things.
First is that the six year old has been or continues to be mean to the dog, teases the dog, etc, or something similar. Either when no is around or no one is looking or at night, etc. Maybe pulls his tail, blows in his face, flicks his ears, etc. something to bother the dog.
Or maybe the dog views him as the weakest, lowest member of the pack and someone he can easily intmidate and scare. By doing that, the dog immediately goes above him a few notches on the pack totem pole.
I would make 2 suggestions: first of all, explain to the 6 year old he is NOT to be mean to the dog or to tease the dog, even when you're not around or looking (ESPECIALLY when you're not around or looking!!). Explain to him that it's wrong and why it's wrong. He and the dog should be friends, eventually the dog will have enough and bite him, and probably seriously injure him.
If that is not the problem, ask if he is nervous or fearful around the dog - and observe for yourself and see if he is acting nervous or fearful around the dog. Again, explain to him that he cannot show fear to the dog, he must act like a pack leader and not show fear, even if he is feeling it. A dog knows when a person is afraid and they take advantage of it. It gives them power over a person.
And finally, you have to take control of the dog and allow it to show absolutely NO AGGRESSION toward the 6 year old or any other of your children. If you are acting nervous or fearful when the 6 year old comes around the dog, the dog will pick up on this and react accordingly. So you yourselves have to act calm when the 6 year old is around. If the dog acts up, stop him immediately, but then go right back to being calm and relaxed. If you are nervous or anxious and just waiting for the next problem, or wondering when the next problem is going to happen, you are MAKING it happen. The dog will pick up on your anxiety or nervousness and it will make him anxious or nervous in response, so again, that causes problems in and of itself.
Again, this is just going on what I read in your email, if I observed the situation with my own 2 eyes, I might see something completely different.
I am sure I could fix this in person in short order. If you would like me to come and help, just let me know. If you'd rather just get my advice, for free, feel free to give me a call on the phone, we can talk about it and I can tell you what to try, if that doesn't work, we can always have me come over later, after you try some things yourself and don't have any luck.
Think about it and let me know, I am always available.
Thanks again for contacting me, and feel free to get back to me anytime.
Brian