Don't be alarmed. Rolling from back to front is a more difficult milestone and your baby may be 5-6 months before he starts rolling from back to front. Babies that don’t get a lot of tummy time may take longer to start rolling over. Try to place your baby on his tummy when he is awake and alert. This will help him with rolling over and other developmental milestones.
Don't compare one kids with other dear.
My elder son roll over at 5 months, started walking in 10th month, 2nd old rolled over at 4, But started walking after 12 month. Every child is different.