Rolling over

Our son is 4.5 months old. He has rolled over from back to belly twice, but hasn’t done it since then. He rolls onto his side easily, and prefers to sleep that way. He used to hate tummy time but now does it like a champ! How can I encourage him to roll over more? I try to position toys in a place that he would need to roll over to see or play with them easily, but it doesn’t seem to phase him. I wish he would figure out that if he wanted to get out of tummy time he could just roll over! Any help would be much appreciated!