If you stop supporting him, what would happen? Would his parents pay? Or the Government?
I'd look into that, as a first step. Obviously nobody is going to be keen taking over payment for someone who has got themselves incarcerated, they'd rather spend their money on themselves, but I think you've done your part now.
Once you've stopped funding him, you'll be able to see whether the change continues. If he's still behaving differently, or if that is a bit of an act. Unfortunately, foreign prisons are masters of teaching how to look like a reformed citizen. A large amount of appeals are lodged based on how different and reformed a person has become during their time inside. You have to keep your wits about you to work out whether he really has changed, or whether it's a cunning act to get you to keep supporting him and help out with his defence, if he needs it.
The fact that your head doesn't really want him to be released is a good indication that this relationship won't survive when, if, he is released.
However, I can understand not wanting to feel like you've just dropped him when he's in prison. I wrote to death row prisoners as a young teen, as part of a school project. When the project ended, they were "encouraged" not to write anymore. Three continued to write to me and I struggled hugely with not responding. They'd done terrible things, that's undeniable, and there was certainly an argument that they deserved to be where they were. But they'd written to me when I'd needed them too, and it felt cold and heartless to drop them. I had to wean myself off writing to them because I couldn't ignore their letters, and I felt evil doing the "I'm too busy with life/exams/boyfriends" that other people seemed to use to end the friendships.
This is probably somewhat the same. A misguided sense of wanting to be able to help, to offer some comfort, to make a difference. So work it to your advantage. Stop the financial support, because that's a good reason for him to lie to you and keep you onside, and you'll never know if he's being genuine whilst you're funding him. You could write to him, to keep him company and still offer some support, or perhaps have phonecalls if he's allowed, but set a schedule that suits you and doesn't interfere with your life too much.
Let yourself grieve both the loss of your relationship and the cheating, because he is still responsible for those, and the fact that he's put you in this position and it'll be a bit of a test of inner strength to get through it unharmed.
Could you do that?