To transfer the input balance from the deceased father's firm to the son's new firm, log into the father's GST account and file Form GST ITC-02 accompanied by a CA certificate. Once filed, log into the son's new GST account to accept the transfer, which moves the balance into his credit ledger. Finally, file Form GST REG-16 on the father's portal to cancel the old registration citing "Death of Sole Proprietor."