Nabokov's son could make a copy of the manuscript and publicly burn it, allow the world to believe that it has been torched and is no longer. But take the original manuscript and secretly lock it away in a safe deposit box where it can be stored for a couple generations. Then, when the son's grandchildren have reached a certain age, allow them to decide whether it should be published or just quietly burned and left alone.

I suggested his grandchildren because they would presumably have enough distance from Nabokov himself to decide which is best for his legacy, something that his unfortunate son just doesn't have.