There are many things that have to happen for a baby to be able to roll over. It is becoming more common for babies to be older before they roll over than it used to be, and much of that is because of positioning. Our babies spend precious little time flat on the floor for the most part. They spend their days in bouncy seats, car seats, propped up by cushions, etc. The necessary skills for rolling cannot develop this way or while in a saucer, etc. Even while we are holding them (which is SO important) they are not developing the necessary skills. I don't know how much time your little one spends on tummy time or lying on his back in the floor, but I would try to increase that. I would also check with his pediatrician; you should always keep the doctor in the loop. Other things to consider: does your little guy reach out and grab objects with both hands? Does he have good head control in sitting? Can he begin chewing soft foods and move them around in his mouth well? Is he babbling often and responding with smiles to people he knows?
You must always look at the big picture when considering development, and opportunity to develop skills is an important consideration. If the opportunity to develop has been there, and it hasn't happened yet, I would be a little more concerned. Otherwise, ample time in the floor should remedy the problem quickly.
K. -- I am a pediatric occupational therapist working part-time, and I stay home with my kids, 8,6, and 3, the rest of the time.