It depends. Does he have a place he will be living when he is released from prison? Will he be living with you? If so, you can use your address or if he has a relative where the rest of his mail is sent, you can use that address. Where do his bank statements go?
You can use the prison address, but you would have to talk to someone there to determine how a refund check would be handled. Remember whatever address you use will be used for any IRS correspondences, so it may not be as easy for him to respond or address any issues as if it was sent to a non prison address.
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