If you are the legal executor of his estate, you can use this IRS link to see if a refund was processed which would be evidence that a tax return had been filed. Otherwise, you can use this link to set up an IRS account or access one that your son had created which would allow you to see if a return was filed.
You can complete form 4868 online and mail it in to receive an extension of time to file a tax return.
I am sorry for your loss.