Relax, your child is well past SIDS danger.
My DS started rolling onto his tummy in his sleep at 3 months! It was horrible, I had quite a few sleepless nights! Not only he preferred tummy, he also managed to sleep face down, resting his forehead on his hands
It seems to be a common theme, babies preferring to sleep on their tummies. However, they say if they are able to find their own comfortable position, they are safe to sleep in whatever way they choose.
However, now it is more important than ever that the bedding is super safe! But I think you already know that, judging by your post:)