I completely agree with the above post that pre-existing disease coverage is available in India, however, after a period of 4-5 years of continuous coverage with the health insurance provider. According to my knowledge, even the best health insurance plan in India, offer coverage for pre-existing disease after a specified time limit rather than covering it from the start of the policy.
As you have mentioned that your son is suffering from congenital heart disease, it will be considered as pre-existing by the health insurance provider and the coverage for the same will be available after a certain time period.