Even with a guilty charge, people still have the right to appeal or the right to sue for wrongful imprisonment. I'm sure it is a bit harder to do and you must have a reason as to why the imprisonment was wrongful - enough to convince a judge/jury.
It's true that if he said he did it by his own admission, it probably wouldn't work. All they would need to do is pull a confession statement and that would be the end of that. He would probably need to seek legal counsel and a lawyer to tell him if that would even work in a courtroom - many times they can tell you if you're just wasting our time. A lawyer doesn't want a case they can't win.
It's hard and painful, I'm sure, to forget an 8 year prison sentence. A lot of nasty things can happen inside prison to people. Many prisoner's have trauma of some kind or PTSD. This, among other things, can make it hard to adjust back into society. All in good time, he can grow in the Lord and take it a day at a time.
They already tried throwing out his confession and it didn't work though.