Each of my children hit the developmental milestones at different rates. My youngest is almost five months and started rolling over a few weeks ago, just from her back to her stomach. The other day she rolled from her stomach to her back, but it was just that one time and she hasn't done it since. I have a co-worker whose son was born two days after she was and he hasn't attempted rolling at all yet. My older daughter, who is almost 3 now, started rolling over when she was six weeks old; my oldest son was closer to six months.
If you're really concerned you could try encouraging him to roll by placing him near a toy that he would have to scoot or roll to get to, and by laying next to him on the floor to play. You can also lay him on his back and sit with him, playing rolly games where you roll him over, to get him used to that motion.