Of course he should go to prison, why is that even a discussion?
"He's old" or "he changed his mind" doesn't bring a single one of those millions of jews, gay people, sinti and roma or political opponents back to life that he helped kill.
Yes, rehabilitation is the central point in the german law system, but look, if he was a cop that killed a black guy or some KKK fucker who helped organize a lynching, you guys would be all over this.
He will go to a german prison, not a U.S. one. He will be treated respectfully as a human being in there, not boxed away and treated like cattle like in a U.S. one. But he deserves his sentence. He helped perpetrating one of the most atrocious mass murders in the history of the human race. He has to atone for that.