Unmodified source for this RPG character: Marvel Universe
Self experimentation using gamma ray enhanced machine, in conjunction with controversial bio-science. The exposure left Brian Banner with the same affliction as his son, but unlike David Brian Banner changes into his hulk form only at night, turning back into his human form at day break.
We know from the later comics that Banner's father was named Brian, the 2003 film states that the father of Bruce Banner is named David (as a nod to the TV show). Since I'm keeping the David Banner name, I cant very have two David's. And so for the Hulk universe we will stick with Brian as the name of David Banner's father.