You really should see an accountant, who can determine which returns need to be whether they should be filed under his social security number or if there needs to be a separate Estate or Trust Tax ID number established. It's money well-spent.
Legal disclaimer: DISCLAIMER This answer is provided for educational purposes only. By using or participating in this site you agree and understand that there is no attorney client privilege between you and the attorney responding. This site cannot be used as a substitute for competent legal advice from a licensed attorney that practices law in the State where this offense is charged and/or wherein the legal issue arises; and, who has experience in the area of law you are asking questions about and with whom you would have an attorney client relationship. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question, or in the State in which your particular issue has arisen.