What plug-in?
Heavy metal is usually really compressed anyhow.
I don't understand why this is in mono though.
It's possible he's used a mono version of a compressor plugin on the master mix. If he used The Tokyo Labs feedback Compressor II, for instance, they have four versions - the Mono 32-bit, the stereo 32-bit, the mono 64-bit and the stereo 64 bit. If you use the mono version on the master bus it will cause the whole mix to output in mono. Worth checking.
To my ear, the whole thing sounds overly bright for the genre...YMMV