Immigrationboards.com

My son is born in UK and I have applied for his registration certificate using Mn1 form.

What is the next action once I get certificate ?

Do I need to apply for Britsih passport ?
What will happen to current Indian passport. Does that needs to be given back before UK passport ?

There is no next action once you receive the certificate.

If you wish, you can apply for a British passport for him. If you don't plan to travel, then there is no need for a passport. Once the Registeration application is granted (and not from British passport issue date), the Indian citizenship is lost and his passport would need to be surrendered to the HCI.