Here's a good link for you:
http://www.babycenter.com/0_developmental-milestones-roll...
He apparently is not yet holding up his torso? Or his neck/head when he is doing tummy time? Can he push up with his forearms?
When doing tummy time with him, perhaps don't try it when he is naturally tired or close to nap time. And try to give him more tummy time....
Is he sitting up or anything?
At 6 months, they usually are rolling over and rolling around. If you are concerned, ask your Pediatrician.
How is his OVERALL development? Is it on par?
The reason I ask is: I once baby-sat a baby like that... at 6-7 months old, she was still NOT rolling over nor even trying... during tummy time she could not do "push ups" nor hold her neck/head up, and she could not sit up either without help or without slumping over. She would just lay down on her back and not try do do much else. She had other developmental delays and concerns though.... not only not rolling over. I told her parents... they did not realize it... and to this day (I saw her once- she is about 5 years old now), she is still delayed in some areas.
NOT saying that this is the same as your son. BUT.... just that there are all kinds of concerns, or not, and all kinds of time-lines as to "when" a baby rolls over.
Bear in mind that there are "milestones" at each age (for which rolling over is a formal milestone) and then there are regular developmental changes and progressions. It is the "milestones" however, which indicate a concern or not, of a child or baby.
Yes, there is also the normal parameters that each baby/child develops differently and on their own... BUT, within that spectrum, there are "delayed" or "normal" ranges. Even if a child is "delayed" in an area... it does not necessarily mean they will be delayed forever, as like that baby I baby-sat. Lots of kids just catch up. My son is "delayed" in talking... but he has now just about almost caught up with his age now... he was previously 6-9 months delayed.
Ultimately, I would ask your Pediatrician.... and go according to your gut. There is nothing wrong with asking your Ped about your concerns... at 6 months, it should be his well-baby check up anyway. They will ask these things.
All the best, your son is probably fine,
Susan