A.,
I wouldn't worry about him rolling over. My daughter HATED being on her tummy and refused to roll over for that reason. IF you put her on her tummy, she screamed and cried until you turned her back over. She learned to crawl (at 7 mos) by leaning forward from a sitting position and getting on all fours that way.
Now she is a healthy, active 10 year old who amazes people with her athletic prowess. You just let him figure things out for himself. Cheer him on when he does work at it. Don't stress yourself out over what the developmental charts say. They are only guidelines. After three kids, I have discovered that the babies can't read those anyway & will do their own thing.
That said, even having a degree in Child Development didn't keep me from worrying over the little stuff from time to time. Enjoy your handsome boy!!
B.