What do you do to help your LO learn to roll?

My LO is just over 4 months old. He will roll from his back to his front (not consistently, but he can do it). He doesn't seem interested in rolling from his back to his front. Any time I lay him on his back to practice, he just kicks his legs and flails his arms while he laughs. Ive tried taking a toy that he's interested in and putting it next to him to encourage him to turn but he will only turn his head to look at it. He makes no attempts to turn his body. How do I encourage him to roll?