There are a couple of ways they can hold him as they don't have life imprisonment. I quote.
In 2008, to fulfill its requirements under the Rome Statute, Norway created a new maximal penalty of 30 years for crimes against humanity.
There is also an indeterminate penalty, called "containment" (Norwegian: forvaring), is set at 21 years imprisonment, and the prisoner is required to serve at least 10 years before becoming eligible for parole.
If the prisoner is still considered dangerous after serving the original sentence, the prisoner can receive up to five years additional containment. If the additional time is served, and the offender is still considered dangerous, a prisoner can continue to receive up to five years additional containment, and this, in theory, could result in actual life imprisonment.
"Containment" is used when the prisoner is deemed a danger to society and there is a great chance of committing violent crimes in the future.[
However, the offender can be paroled or released at any time if it is determined that the offender is no longer a danger to society.
So now we know.