Does it seem like your baby wants to roll? Is he wiggling around while on his back, flopping his hands from side to side? Can he already roll from his tummy time position to his back? You can get in on the action with him by helping him practice. If it is nice weather, you can go outside and put a blanket down on a slight incline which will make it easier for him to roll.
Indoors you can gently roll baby from side to side encouraging the motion and generally enjoying some fun time together. Remember, infant stimulation is about time spent together and interaction; don't worry about whether your baby is actually rolling or not. He will be moving around soon enough!