I think, if someone told him it would have been after he started dating Peyton. After all, he seemed very surprised by her confession of love.
And while I think he was often a crappy boyfriend to Brooke, I don't think this was really one of his bad things. He thought he knew what their fight was about and tried to explain. He realised he shared something she didn't know and than tried to fix their relationship. The fight she had with Peyton took the backseat to fixing him and Brooke for him, I think. Which isn't too weird. The fight had not involved him (directly), so it wasn't a priority anymore once he started arguing with Brooke.
I hope this made sense, it's the middle of the night and way past my bedtime