Just tell him you'll make the holes and find the cable at your normal hourly rate. It wasn't part of your original scope of works so it's an extra.
Just for future it's imperative the everything that's agreed or requested by the client, no matter how small and inconsequential it may seem at the time, is in writing. If necessary email him that evening stating every decision he's made during your meeting with a request for clarification within 24 hrs if there's something he disagrees with. Unfortunately verbal instructions and agreements don't cut the mustard, if there's ever any comeback or disagreements later on.
Just for future it's imperative the everything that's agreed or requested by the client, no matter how small and inconsequential it may seem at the time, is in writing. If necessary email him that evening stating every decision he's made during your meeting with a request for clarification within 24 hrs if there's something he disagrees with. Unfortunately verbal instructions and agreements don't cut the mustard, if there's ever any comeback or disagreements later on.