S.H.
It's okay...it's seems he's gotten to the age whereby he can put himself into a "comfy" sleep position by himself...a position that he likes to sleep in.
My son and daughter did the same thing... it's fine. As long as he is able to flip himself over in both directions by now...he's also standing already, so his gross motor skills seems on par.
Just let him sleep on his tummy...most kids sleep better this way, and he is past the 6 month old mark...so that's good.
I was so happy when my kids reached this point in mobility...then, they "can" put themselves to sleep better...and it helps their self-soothing skills too.
All the best,
~Susan