That may or may not work for a whale ON LAND. Whales, however, live in the ocean, and thus you'd have water resistance to fight as well to get the head to move fast enough for the brain to hit the inside of the skull that hard. The ratio of brain to CSF to subarachnoid matter may not be similar enough to scale a concussion this way. Most comparisons are done with land mammals.
I'm also not sure about your math. For convenience, let's assume accelerating a whale's skull into its brain does about the same damage at the same rate. I think it makes sense to keep the acceleration number constant, rather than trying to scale the force by weight and size, which doesn't really work when you cancel out the units.
Human concussions happen at about 95 Gs. That's about a 931m/s/s acceleration. Since force is mass times acceleration, we then need the mass of the whale's head, which you estimate at roughly 20,000 kg (assuming you were using metric tons). That makes the force about 18. 6 million newtons, which rounds to about 18,600 metric tons of force. For reference, that's about five and a half times the force needed to get a space shuttle off the ground.
Holy crap. Can that be right? It's late. Someone smarter come and check my physics.