All I can say is that I completely sympathize! We had a dog named Sabre. Half black lab, half border collie. She did the same thing your boy does. She would sit there all pretty and inviting, even run up to people and nudge their hands to be petted. She'd let them pet about one time and then bite. Not an attacking or vicious bite. Just enough to let them know she was the queen. No matter what we did, she never grew out of it. That was just her personality. She protected our family like none other. I refused to put a muzzle on her. I did it one time to give her a bath (she hated baths) after she got sprayed by a skunk. Never again though. It was awful for her. Sadly, we had to put her down 3 weeks ago. She was a little over 20 yrs. old.
Everyone that knows us loved her. Regardless of her habit. They all knew it and came to appreciate it. I'm not sure if that's what your little guy is going through but it really sounds like he wants to protect you. I'm not a dog trainer, so I have no advice. Just wanted to offer up some sympathy.