First of all, even if you can't think of what to say, I'd recommend you say something. "I'm terribly sorry for your loss" is a good general-purpose catchall.

But also... I think I would have valued the opportunity to hear from someone who knew my father and had a perspective similar to mine. Nothing profound, just "I knew him, this is what I thought he was like, this was our relationship, these are my feelings about his death" in a way that would invite me to do the same, as a peer-to-peer thing rather than a mourner-confidant thing. I'm not sure if that makes any sense, but I hope it does.