My baby is 2.5 months. How to train him to roll over? Any game or play?

2.5 months is too early to get bothered about rolling :) You can make it fun by putting your baby on his back and then standing in a direction that would require him to roll over. Mama/Papa/Grandpa/Grandma or any other loved ones would be able to do that. You can also put his favorite toy far from him and encourage him to go grab it. My kid had a favorite book and we used that as a bait :)