First you need to enter all of his information, including his income and tax documents, into TurboTax as you would a normal return.  Answer any questions about incarceration as appropriate.

You can sign his return for him, but you need a valid power of attorney, which your son has to sign.  And, there are a few steps you need to follow.

The power of attorney form, required by the IRS, Form 2848, can be found HERE. 

If you paper-file your son's return through the mail, simply attach the signed Form 2848 to the complete return.

If you eFile his return, however, the process is a bit more complicated.  You'll eFile the return as normal through TurboTax.

THEN, you'll need to send both the signed Form 2848 and the IRS' Form 8453, U.S. Individual Income Tax Transmittal for an IRS e-file Return to the IRS.  On Form 8453, you'll check the second checkbox, for "Form 2848, Power of Attorney and Declaration of Representative."  There's no signature requirement on Form 8453.  The mailing address for these items is on page 2 of Form 8453.

CAUTION:  If you've eFiled your son return, do not include a copy of the return with the Form 2848/8453 package you're sending.  It could become detached from the forms you're sending in, the IRS could process it as an original return, and all kinds of problems could ensue.