You have three options for this scenario:
1) You can mail him an IRS Form 2848 which is the power of attorney form that will delegate to you the ability to file a return for him. Then you would print the tax return and sign it and mail it to the IRS with Form 2848.
2) You can complete the tax return, print the return and take it to him in prison to sign in ink and then file by mail.
3) If you would like to e-file the return, then mail Form 8453 along with the Signed Form 2848 to the IRS within 3 business days after you have received acknowledgement from TurboTax that the IRS has accepted your electronically filed tax return. Form 8453 has a specific box to check if you are attaching a POA indicating that the individual has authority to sign the tax return.
