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Use the name on his most recent Social Security card.  When you send a return electronically, the IRS does a file matching based on the name, birthdate, and social security number of each individual reported on the tax return.  To file electronically, you want to use the name and social as registered with the Social Security Administration.  If the information you enter does not match the IRS records, the return is rejected electronically and the return has to be fixed and resent.

{Edited 1/27/18: 9:32 PST}