Harikrishnan, being parents, both of you have taken a revolutionary step to come out of ghetto system of caste. However, your son is free after maturity to follow the footsteps of his parents or revert back to the devisive ghetto system.
Mentioning caste is not mandatory in school admission forms and I know many parents filling Indian even in caste and schools never objected for this. The caste is necessary only to claim concessions/reservations/minority status etc., in future as a record.
In fact, there is no \ that a person need to mention caste or religion while getting admission to an educational institution or a similar purpose unless the person or his guardian/parent seeks to get any advantage based on that classificatio.