So let him buy his food from the commissary. The prison doesn’t serve potatoes? You can live off potatoes alone for a long time. Is there juice, cereal, rice, or beans? I find it hard to believe there isn’t. He’s clearly exaggerating the limits of his diet.
It’s jail. You don’t get to go where you want, do what you want, wear what you want, or eat what you want. You don’t get to make choices about your life. That is part of the punishment.