My 21-years old son is a dependent on our tax return. He earned $11,000 as a summer intern that was not taxable. How can we get his tax withheld refunded? Thanks!

He has to file his own tax return and enter the W-2 received on the return.  On his return he will have to indicate that he can be claimed as a dependent

You never enter a dependent's earned income on your tax return.

Your dependent can get a refund of amounts from boxes 2 or 17 of his W-2 for federal and state tax withheld..  He will not get back anything withheld for Social Security or Medicare from boxes 4 or 6. 

MY DEPENDENT HAD A JOB

If your dependent has a W-2 for his after-school job, summer job, etc. you do not include the information on your own return. You can still claim your child as a dependent on your own return.  He/she can file his own return for a refund of some of his withheld wages (he won’t get back anything for Social Security or Medicare), but MUST indicate on it that he can be claimed as a dependent on someone else’s return.  (Supervise this closely or prepare it for him!)

If your dependent’s earnings were over $400 and were reported on a 1099Misc or 1099NEC then he must file a return and pay self-employment tax for Social Security and Medicare.

You might also want to use free software from the IRS Free File versions:

https://apps.irs.gov/app/freeFile/

As for his standard deduction amount:

The standard deduction amount for an individual who may be claimed as a dependent by another taxpayer cannot exceed the greater of $1,100 or the sum of $400 and the individual’s earned income (not to exceed the regular standard deduction amount).    You will see his standard deduction on line 12 of his Form 1040.

He has to file his own return to get the withholding back.   Be sure he marks his return to say he is a dependent on someone else's return.  Did he make less than the Standard Deduction?  How much did he make?

The Standard Deduction for an individual for whom an exemption can be claimed on another person's tax return is limited to the greater of:

…..$1,150 or

…..Your earned income for the year + $400 (but not more than 12,950))

So you would get a minimum of $1,150.

See Table 8 Standard Deduction worksheet for Dependents on Page 26

https://www.irs.gov/pub/irs-pdf/p501.pdf

Or the 1040 worksheet on page 34

https://www.irs.gov/pub/irs-pdf/i1040gi.pdf

How to start a new online return

Https://ttlc.intuit.com/questions/1894512-how-do-i-start-another-return-in-turbotax-online 

Be sure on their return they check the box that says they can be claimed on someone else’s return.  If their only income is W2 and under 12,950 they do not have to file a return except to get back any withholding taken out.

Filing requirements for a dependent

https://ttlc.intuit.com/community/children-dependents/help/do-i-need-to-file-my-own-taxes-if-i-m-a-dependent/00/26111

And see IRS Publication 501

https://www.irs.gov/pub/irs-pdf/p501.pdf

Really appreciate all the quick replies, which are super helpful and completely solved my problem. Thanks so much!!