I dunno, he should have got you onto fish ...but sometimes ish just doesn't work out even if you're a seasoned pro on your home lake/river.
Honest, I would say tip him 20% or more if he has a newer rig, and he isn't bitter about the freebie trip. If he's bitter, end the trip early and say thanks.
If you STILL don't get anything, but had fun and it was just bad timing yet again, it's your call. I'd ask to switch target species just to salvage the day, and give him 15%.
My opinion only, opinions vary.