Your birth certificate is what should help you in such cases to prove your nativity. But if you were born in Saudi Arabia, the place of birth will be Saudi Arabia and not Kerala. Your birth certificate issued by the Doctor will be a proof. You can be considered a native of Kerala by the virtue of your parents belonging to Kerala but your birth place will always be Saudi Arabia. Your parents school certificate is another document that can help you with a nativity certificate. Please check with local tehsildar for the necessary steps in regards to this.