He will use his SSN on his tax return and every tax document from the time he received it. If he did not already, he should contact the IRS, and let them know that he now has an SSN instead of the ITIN so they can link or match them.
From the IRS, You can't have both an ITIN and an SSN.
"Once you get an SSN:
1. Stop using your ITIN
Use your SSN for tax purposes.
2. Notify us
When you get an SSN, notify the IRS. We'll combine all your tax records under your SSN. If you don't notify us, you may not receive credit for all wages paid and taxes withheld. This could reduce the amount of any refund due.
You can notify us by mail or at a local IRS office.
How to notify us by mail
Write a letter stating you were assigned an SSN and want your tax records combined.
Include your:
Mail the letter to:
Internal Revenue Service
Austin, TX 73301-0057
We'll void your ITIN and associate all prior tax information filed under the ITIN with the SSN."
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