My girls have the same problem. Their arms just fly all over the place when they sleep and because of it they won't sleep for more than 20 min on their backs. they will only sleep on their tummy. I've read numerous articles saying babies should only sleep on their backs to avoid SIDS, but if the baby can pick up his head on his own when laying on his tummy, then maybe you should try that? Just make sure there are no loose articles and that the sheet fits very very tightly. My girls are now almost 3 months and sleep most of the night.