I agree to the above your puppy is asserting its dominence, and finding it's place in your home (pack) ... regardless of breed, it is up to you to point out his place in your pack

I used to raise horses, and one of the first things we did with a newborn foal (and this may sound cruel), was to lightly pin it to the ground, enough that it knew that a human could hold it down, but not enough to hurt it ... guess what? every foal of mine grew up thinking us humans were bigger & stronger & they respected us for life ... similar philosophy on dominance in your pack ... who's the boss? and who do you want to be the boss?

find a trainer who can help you one on one to correct this now, or it is a battle this dog will have for life, regardless of his owners, but you must find a trainer who can watch it happen ... us net friends around the country cannot help you with the exact hows to fix the issue