I'm a little worried OP that you work with offenders given what you've said.
I worked with offenders for a few years and if you're in a role working with this group you should be aware of the pitfalls that so many women (and workers) fall into but you don't seem to have much insight.
Prison is fucking awful, both here and abroad. It is so important to inmates to have someone outside waiting for them/loving them. Prison is such an abnormal environment that they need hope, someone outside who loves them when their day to day life is so abnormal.
They will promise the earth, will soul - search and promise and examine their previous behaviour and life. And despite what a lot of people on this thread think, the majority of them do mean it.
But they only mean it at THAT time because they're in an extremely stressful situation with no opportunity for real emotional connection to anyone. So they cling on to that, their need for that connection consumes them so they really think it'll all be different, THEY will be different and it'll all be ok and lovely when they come out.
They need to believe that. That's what keeps them going. But it's a fantasy created by an abnormal environment and extreme psychosocial stress.
And they come out and it's all brilliant for a short time. But they're in the real world now and not under so much stress so very quickly, that person they relied on in prison really isn't that important anymore. They're not waiting all day or all week for a phone call or a letter with nothing else to look forward to - they're out now and there are very, very many fish in the sea in the free world. So many temptations and opportunities.
So they revert to the behaviour they displayed before prison.
Put your life on hold for years if you like but I guarantee he'll be his old, selfish and unfaithful self very quickly.
Of course you could alternatively be the kind of woman who works with offenders because you find offenders exciting (there are many). The idea that he's capable of awful things but you may be able to tame him. The romance of waiting for him, the passionate letters and the tortured scenes of love in prison visiting rooms.
The fact that only YOU really understands him and why he does the things he does and if you love him enough he'll change and he'll tell everyone your love put him on the straight and narrow.
Both types of women in this post are mugs and on a hiding to nothing.