A.K.
rather than trying to keep him from rolling over I would help him learn to roll back. do lots of tummy time to help him get use to being on his belly and to help him strengthen his upper body. I use to lay on my back w/my knees bent and chins in the air (parallel to the ground) then I would lay my daughter on my chins hold her hands and rock. Its a great work out for both of you and my daughter loved it.
good luck