That is a great documentary.Better yet, he should sit down and watch the following:
Heavy: The Story of Metal
The guy goes seriously into nearly every genre of metal, from pre-metal to the split between the US and UK, NWOBHM, Thrash, Prog, Glam, Grunge, Nu, Power, all the way up to Metalcore.
And the one thing mentioned specifically is where it started, and who started it.
BL.
P.S. Black Sabbath is Heavy Metal.