Going nowhere fast

Harry is doing my nut in at the moment. He has mastered the art of rolling over onto his stomach and is desperate to crawl. But he has not realised that he has to have his feet on the ground to push with. At present he has his knees bent and his feet up near his bum, so all he is achieving is rocking back and to on his tummy. After a while he gets really frustrated with himself and starts crying, so I put him on his back and he cries even more because I have turned him over and he wants to try and crawl. You can't win !!!!