Sleeping on tummy

Oliver was very active in his sleep recently. He would turn to his sides, moved up and down the bed. Having a bolster was no help at all. Three days ago I found him sleeping peacefully on his tummy. Why now? Months ago, I was trying so hard to get Oliver to sleep on his tummy to prevent him from having flat head. But he just couldn't sleep on his tummy. Now that he can, there is no hope for his flat head already.