Jack Rolling Over

I've been trying to upload a video of Jack rolling over but it is not working right now. Anyways, Jack rolled over for the first time on November 14th (his five month old birthday). Since then he had not rolled over. Two days ago I put Jack on his back on his activity mat. I then went into the kitchen. When I checked on him a couple minutes later he was on his tummy! Yesterday he rolled over about 1o times! He is able to roll onto his tummy, but doesn't quiet know how to get back on his back.