Dan Houser described him as having a "powerful" voice. Official Playstation 2 Magazine USA, Issue 86 was the mag that had the info, an extract from it is below:
"Actually, it was through DJ Pooh. He's been a producer forever...I was introduced, then the Rockstar family picked me up...I didn't know that Dan was listening, and I guess he was getting the idea about me being the voice right then."
Thanks to GTAGaming for the heads up