This one hurts, and it deserves a straight answer rather than a reassuring one.
When an inmate tells the person they love to step back, it is almost never actually about wanting distance. It is usually about pain they do not know how to manage. Someone staring down a long sentence starts doing a particular kind of math in their head. They look at the time they have left, they look at the person on the outside living a full life that keeps moving forward, and they convince themselves that the ending is already written. That you will leave eventually anyway. That holding on is just prolonging something inevitable and making it hurt more when it finally happens.
So they try to control the only thing they feel they can control, which is the ending. Pushing you away first feels less terrifying than waiting to be left.
It is also possible he is going through something inside that he does not want you to see. A hard stretch, a situation with other inmates, a mental health moment he is not ready to talk about. Telling you to take space can be a way of buying time without having to explain anything.
The suggestion here is worth taking seriously but not permanently. Give it a few weeks the way he asked. Let the pressure release a little. Then re-engage gently, not with a conversation about the relationship, but with something simple and warm. A magazine subscription is actually a smart move here because it keeps arriving on its own schedule, once a month, with no demand attached to it. Every issue that shows up is a quiet reminder that someone out there is still thinking about him without requiring anything back.
Sometimes that kind of steady, low-pressure presence is what breaks through when words cannot.