I've heard that most babies learn tummy-to-back before back-to-tummy because they find it easier to push up with their arms.
DD, like your LO, was quite the opposite. She did back-to-tummy first. We didn't do anything to get her to roll from tummy-to-back, she just eventually figured it out.
You might try showing him how it is done, by demonstrating yourself or demonstrating by rolling him in the tummy-to-back direction so he gets the idea of what is supposed to happen.