I don't want to get in any trouble so I want to make sure it's ok before I do anything. Honestly, I'm at a loss as how to help him get the next check and Google didn't help me too much.
Yes, he would need to file a 2020 Tax Return, If he did not receive either the 1st or 2nd Stimulus Payments. Enter this info in the Federal Review section of his return.
He is now eligible to claim those payments to qualify for a Recovery Rebate Credit when filing a 2020 return.
There hasn't been a 3rd Stimulus Payment authorized yet.
Click this link for more info on the Stimulus Payments for Incarcerated Persons.
No, you are not legally allowed to file a tax return on his behalf. Although you can help your friend with his return by compiling all of his documents and completing the necessary forms, you cannot sign and submit the return on his behalf. You could, however, send the return to him for review and signature and then once you receive it back, you can mail it to the IRS and state for him.
If your friend is willing, he can sign off on you being a Power of Attorney, which will then allow you to file his taxes without his review and signature. You would do this by filling out Form 2848.
Please follow this link to get the form on the IRS website Form 2848.