Hi there and thanks for the post.
This is a normal puppy response. It is a form of submission as he sees you as the boss. They do grow out of it but it will take time.
When he does roll on his back turn away, don't talk to him, don't pet him. You have to wait until he is being confident again to show him attention and praise. This will help reinforce confident behaviour not submission.
Good luck and if the problem doesn't solve ask again and we will go from there
Todd