Trying to roll over

Each day now ... David gets closer and closer to rolling over. He certainly loves to rotate or flip his hips so he can change direction. Many times I'll look in on him in the pack-n-play and he's managed to turn his body 180degrees. I'll look in again and he's back to where he started. But now when I put him on his stomach for tummy time he starts to lift his rear to push over. But can't quite seem to get his arm out of the way to complete the roll. He is even trying to roll from his back to stomach too. But the same instance is occuring ... he can't seem to get his arm out of the way to complete the roll.

He's growing and getting so big ... but he's still not even 4 months old yet!!