You cannot find out if someone else entered his SS number on their tax return and the IRS will not tell you.
You will need to verify that the SS number and name are correct from the SS administration since that is where the IRS receives the SS information.
To prevent this rejection next year, call the Social Security Administration (SSA) at 1-800-772-1213 to verify their records (this is where the IRS database pulls information from).
If the IRS continues to reject the tax return after you have verified the name and the SS number are entered exactly as shown on his SS card then you will have to print and mail your tax return.
See this TurboTax support FAQ for how to print a tax return for mailing when using the online editions - https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online...
