Most people come out of jail fat because the nutrition is garbage and their only comforts are the junk food from commissary, chris especially will gain weight because all he will do is shit himself while seated on his prayer rug while playing solitaire and doodling cartoons. even if he attempted to work out his diet would still be abysmal and he would give up any physical activity before the day was done. He would need to be housed with a cell mate who got him onto a routine and forced him to keep up with it daily while getting protein powder and a restricted diet from commissary instead of soft drinks and junk food. Since he is segregated that obviously won't happen and if chris does ever actually go to an actual prison or jail where he is housed with other inmates the only people interacting with him will be people who will be scamming him out of those white knight commissary bucks and whatever items they can trick him into paying for.
If Chris does end in in big boy prison I think his situation won't change too much. Part of any deal Heilburg reaches if it does come down to some felony prison time, could include keeping Chris away from other inmates as much possible. It'll be difficult yes but not impossible. Really if Chris is put back in isolated/protected custody that means no cellmate so that's a plus, really the only times he'd be at risk for running afoul of other inmates is when the prison does a roll call and calls all inmates to the yard, during transfers if chris needs to be moved someplace, and of course the showers. Which while not the raping holes media paints them as (some more modern ones have stalls and doors with strong locks like any pool shower and the inmates are closely monitored) incidents do still happen in them.