I dunno about their system but in the UK you can't appeal 5 years after sentence unless significant new evidence has appeared which brings the conviction into question. I think they'll go some to find evidence that made it look like somebody else did it given he openly admitted to doing it, they have numerous witnesses among those who survived who say he did it and a wealth of forensic evidence pointing to his guilt.
He's not getting out of jail, ever. The Norwegian population to a man and woman never want to see his face again, ever. Any appeal he might attempt isn't even going to get anywhere near a court room. Even if he did want to further persecute the families of the people he killed - and there's no sign at all that he does, he shows no malice towards the individuals he killed, only the society which they were part of - the Norwegians won't let him do it.
Jon