Rolling over

My son is going to be 5 months and he still has yet to roll over from belly to back AND from back to belly. He is just so content to just lay there. When he does try, he gets frustrated. We have tried laying on the floor next to him to see if he would roll over and also placing noisy toys on the sides of him to try and get him to roll... should I be concerned or is there something else I could be trying for him??