Submissive puppy?

Hi there, I’ve got a 4 month old lab pup he’s absolutely awesome has picked up on commands quite well and is really well behaved. He’s not scared of me or anything for that matter loud noises vacuums other animals he dosent ever flinch. But every time I try to give him affection he pins his ears back wags his tail low and fast and then rolls over. Every single time I go to touch him he turns into a wet noodle. It’s making it very difficult to teach him to lie down, shake etc as he won’t really focus without just flopping onto his back because I’m close to him. Is there anything causing this or anything I can do to prevent this behaviour