Is it ok to let my baby sleep like this?? (Please give reasons why)

So every since my baby could comfortably roll from front to back and back to front he always sleeps lying on his front with him knees up and bump in the air. I have tried rolling him over but he just cries until he gets in that position again. I always put him to bed on his back and with his blanket tucked below his arms. He is 6 months old and I'm moving him from the Moses basket beside my bed into a cot in his own room soon so that's why I'm worried. He always puts his head to the side but I constantly check on him during the night and I'm afraid that when he is alone that something will go wrong. What do you think?