It's his knucklehead interpretation of the relativistic energy equation.In exactly what universe is this equation true?

In geometric units

E^2 = m^2 + p^2

In conventional units

E^2_conv = (mc^2)^2 + p^2_conv c^2

And no he can't add anything to the derivation as you already know. He could learn how the derivation is achieved but I'm thinking that's out of the question. When the momentum is 0 the invariant mass equals the invariant energy and this is what led to the Manhattan project. The mass is invariant by definition [some discussion required to formulate a proof] and the momentum and energy are constants of the motion. He could find the derivation in chapter 1, Speeding, of the text Exploring Black Holes.

http://www.eftaylor.com/download.html#general_relativity

Choose chapter 1 Speeding to download.