I assume you mean "actually a Nazi" in the sense of "what he a true believer in Nazi ideology" as opposed to "was he a member of the Nazi party" because the answer to the latter is clearly and well-known to be yes. Von Braun joined the NSDAP in 1937 and jointed the SS in 1940.

At some point one needs to ask, what, exactly, is the criteria people are looking for? And why? There is no doubt that Von Braun did everything he could during the war to build superweapons for the Nazis, and to use them against civilians in Allied countries, and was aware that the rockets were being assembled by slave labor. By most "moral" criteria that would make him a Nazi — it is hard to imagine concluding he was not a Nazi except for the fact that the US then hired him to run its own rocket program and he was friends with Walt Disney.

Von Braun later claimed he was at best, just being patriotic, and, at worst, just being an opportunist. In other words, he never claimed to be especially interested in "political" questions. Even if we take him at his word for that (I think one can choose to or not), does that make him any better? He wasn't just building rockets, he was building rocket weapons for Hitler — there is no way that he was not aware of that, so what does it matter?

Before people ask, "what option did he have" — there were many scientists and engineers in Germany who, during the war, chose either not to work on military projects or found ways to drag their feet on them. Von Braun did the opposite of this. To be sure, there were scientists who worked for the Allies who also made weapons that killed civilians, but they never made any beef about the fact that they were doing so or that they were doing it to support the Allied cause. Life is complicated, politics is complicated, war is especially complicated, esp. for those in Germany, but the idea that one could come to any conclusion other than Von Braun being an enthusiastic enabler of Germany war aims, and a profiter off of them, seems impossible to me, except out of some misguided desire to preserve him as an acceptable "hero" or a belief that somehow our lives as people are separate from the causes we devote them to.