Hi My 5 month old son has started rolling on to his tummy since last month or so. When I try to put him to sleep, sometimes he immediately tries to roll over. When I try to put him on his back, he cries a lot. Wanted to check if it is OK if we let him roll over immediately when he is put to sleep.

This is very normal. Most babies will be rolling & preferring to sleep on their tummy, between 4-6 months.
Have you moved him from basinet to cot?

Ensure the following:
Cot mattress is firm & flat.
Nothing loose or soft in the cot
Always place baby on back for sleep.

As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.