Labrador puppy biting

Question
Hi, I have a male labrador retriever who will be 5 months old this Monday (July 17th). I got him from an American lab breeder, met both the parents and was very pleased. They were both gentle, affectionate, and seeminly well behaved dogs. Now on the contrary, my puppy Henry is anything butt affectionate. Sometimes he'll come up to me, seemingly wanting to be petted or played with and he'll put his head to the ground (top side down) like he's going to tumble. Then he'll complete the roll on his back and flail his head around, mouth open and snapping, sometimes barking too. When he was younger, he did this with a toy in his mouth, but it seems that now he does it in aggression. Sometimes he'll even just come and sit by me, so I will start to pet him, then he gets excited and flails his head around, again, mouth opened, sometimes barking. When I tell him no, it's like he is back talking; he will stop doing whatever it is I say no to, but barks angrily looking at me, if I don't hold his mouth shut, he snaps his teeth looking me directly in the eye with that same mean stare. I have followed all the instructions about being the pack-leader, but can't seem to get him to stop this behavior, and since he is so young I want to make sure I am using the right technique. I don't want to ruin him by being too harsh, or be overly soft. He is clearly a smart dog, was easy to train to sit, leave it, and heel, but he will NOT lie down unless I lure him into position with a treat (seems like another dominance thing to me). I would really appreciate some advice in how to handle him. Hope to hear from you soon, Thanks.

Answer
I don't think you are getting through to him that you are top dog.  Some Labs can be very difficult.  Find a copy of Marley and Me and read it.  It will be no help, just an illustration of what happens if you don't provide good leadership.

That rolling over reminds of Prince.  He would go through having the top of his head on the floor while still standing on his hind legs, then finally flopping clear on his back, demanding a belly rub.  Here are some exercises that should help:

''Elevation for small puppies:  Sit on the floor and gently put your hands around your pup's middle, below his front legs, and lift him up.  He is facing you.  Hold him for 15 seconds.  Repeat until he no longer struggles.  If he is past 10-12 weeks, lift his front feet off the ground, but don't pick him up.  

Cradling for small puppies:  Hold your puppy gently on his back, as you would cradle a small baby.  If he struggles, hold him firmly until he quiets for 10-15 seconds.  With larger pups, you can do this as your sit on the floor, with your pup between your legs.

Quiet lying down:  Place your pup on the floor on his side, with all 4 legs pointing away from you.  Use your hands on his neck/shoulder area and middle, to hold him in this position.  When he is quiet, praise him.  Lengthen the time that you keep him quietly in this position.  When he accepts this position well, handle his paws and muzzle, while keeping him quiet.''

The quotes mean this isn't my original work.  It is copied from my Puppy Raising Manual.  I have long used these or minor variations of them, and they are very effective.  You may want to give him a belly rub while he is on his back too.  Helps bonding.  There is a big difference between him rolling over and demanding a belly rub, and you choosing a time to roll him over and rub his belly.  The latter cements your place as pack leader.

Also, see http://www.dogbreedinfo.com./topdogrules.htm