Baby hasn't rolled over or pushed up with his arms yet

My baby will be 4 months on the 24th and I've been reading alot of babies around his age are already rolling over and seeing pics of them lifting up with their arms on their belly. Mine hasn't done any of this yet. When I do tummy time with him, he doesn't do anything but cry and just lay flat. He can hold his head up pretty good but that's it. Should I be concerned he's not doing these things yet and what can I do to help him learn these things?