We've been encouraging him to roll for some time now, with little success.
The other day we put him on the floor to play while we were finishing lunch. I asked Mike to lean over and check on him and Mike says "How did you lay him down?" I responded that I had laid him perpendicular to the coffee table. Mike was like "On his back?" Sure enough, while we weren't looking, he had rolled over. He still doesn't do it often, and once he rolls over he gets stuck, but he can do it!
Good job little buddy!