This is really unlikely but if as you say, he managed to turn his life totally around and Serco were OK with that, they could decide to employ him. As I have said before, a DBS disclosure just shows what is on the record and it's up to the employer what they think about it. They might even think that the experience would help him relate to prisoners better.
Which is an important point. Certainly I have come across prison officers of all kinds - those who just don't like prisoners and no way would I go to them with a question, and others who are nice.
Just to name a few, there was Mr Clark who called me Flash (because of Flash Gordon and that's my surname). He took the trouble to have a quick word "I'm not getting at you, it just helps me remember". Fine, I said, my Dad got called that at school. And on the induction wing of 80+ prisoners where you have prisoners moving in and out fairly fast, usually nobody's there for more than a few weeks, anything that helps to remember who everybody is is no bad thing! Mr McMurragh would always be nice to you if you were nice to him. Then there was Mr Wickham... a lot of prisoners didn't like him because he wouldn't stand for any nonsense, but he was always nice as pie to me because I wouldn't throw a paddy like some little kid if I didn't get what I want. It was worth it to him to explain "no I can't do that but I can do this for you". But Mr Perry... anyone who throws a toilet roll at me when I went to ask for one is not going to be my friend.
Bear in mind that Serco is a private company and HM Prison & Probation Service itself might be less forgiving. The vast majority of prisons in the UK are run directly by HM Prison & Probation Service - Serco only runs a few of them. I've been in three prisons and none of them were privatised. Serco also do those dark-windowed vans for transporting prisoners and that's the only time I've come across them.
To answer the actual question, yes, that is obviously possible as you've seen it.
What is CJS? It helps to explain your abbreviations.
Edit - You said "I’ve met people telling me they prefer working for offenders in the community". I can well believe that - you haven't done anything bad enough to be locked up and are likely to be less trouble. I was in one prison that had a YOI next door and there were officers who worked in both. A group of us asked an officer once which she preferred and she said definitely the adult side. It's only the worst of the worst young people who end up in a YOI, because courts try to keep young people out of imprisonment, and they have a lot more problems with violence.
Some not very scientific analysis just from observation of other prisoners led me to the conclusion that something happens about the age of 30 where they start to behave better and realise that hitting people doesn't get you anywhere. And if they're sharing a cell, there IS someone else living in it and they can't have their own way all the time. Of course this is in a male prison - it might be different in a prison for women.
In another prison I was in, the wings definitely varied and after induction, I was put on one full of older prisoners and lifers. "How did you manage that?" I was asked by prisoners who knew it was a nice wing. I have no idea, nobody ever told me, but my guess is it's the best place for someone who just wants a quiet life. It meant being in with several murderers, which alarmed my Mum, but any murderer who has behaved well and worked their way down to Category C escape risk isn't going to be too much trouble. Plus putting all the troublemakers together somewhere else keeps them out of OUR way and enables the prison to concentrate resources there.
Prison life really can end up being what you make it! Act like a spoilt brat and you end up with all the other spoilt brats.
Yes, it makes sense to me that having been "on the receiving end", that could be a positive because you would know how you want to be treated. There's just the public and political view of that, which wouldn't understand it and certainly wouldn't stand for anyone with a conviction being a police officer.