As the executor of the estate you should go back and file the back tax returns. It is some what of a mute point since the estate does not have any funds to pay the taxes that might be due. It is also possible that he might be due a refund. You can request a transcript from the IRS to see what information returns were reported on him and use this as a starting point for filing the returns. Then fill in the blanks with any other documents or knowledge that you have. Then file the returns. The life insurance should have beneficiaries and this money should be out of the hands of the IRS.
Any individual seeking legal advice for their own situation should retain their own legal counsel as this blog provides information that is general in nature and not specific to any person's unique situation. Circular 230 Disclaimer - Advice given in this Blog cannot be used to eliminate penalties with the IRS or any other governmental agency.
First, please accept my condolences on your loss.
Under Massachusetts law, the executor can only pay debts with assets that could be deemed to be part of the probate estate. The answers to your questions are fact-based. I would want to know more about his sources of income -- Social Security? Something else? Was there any real estate sold during those years? It's also entirely possible that your father did not have sufficient income to pay taxes at all.
Please feel free to contact my office if we can be of assistance at 781-433-8665.