Life insurance claim rejected - non-disclosure of pre-existing disease

My son passed away 4 months ago due to heart attack. He had a term insurance policy of 75 lakhs crores from SBI Life which was bought 7 years ago. When we filed the death claim, the insurance company rejected it saying non-disclosure of pre-existing disease. We have submitted all documents including death certificate, policy papers, hospital records, and FIR. The insurance company is not giving a proper explanation. We are a middle-class family and this claim amount is very important for us. What can we do now? Can we file a complaint with IRDAI? Should we hire a lawyer? Has anyone successfully challenged a claim rejection? Please guide us.