My little one rolled over at about 4 weeks from back to tummy, but she only did it once. I think she surprised herself, too. I don't necessarily think it was very purposeful, and she only did it once. She rolled up onto her sides a lot though, which she learned from nursing side-lying. When she hit 3 months and wanted to roll over for a reason (to get a toy, or see something better) she did it all the time. I think once her awareness extended to things that weren't necessarily in her direct line of vision, she started using her muscles. So, long story short, I think babies are physically capable of more than they do a lot of times, but have to have the motivation to do them. So don't leave him anywhere he can roll off of.