I think the issue with idolatry is simply that from his POV, the Eucharist is not really God. If that were true, than the way Catholics treat it really would be idolatry, by definition - worshiping something that is not God. So given his views, it is actually a sensible conclusion for him to draw.
I would just say, if it comes up, that Catholics really do think God is present in the Eucharist, and if that is so, than their attitude is not idolatry, but only proper.
So the issue in question is really why you differ on your beliefs about the Eucharist. If he’s a reasonable guy, he will likely see the logic. Pretty much all Christians agree that it is really wrong to worship something other than God, and that to do so is idolatry. All agree that it is also wrong to treat God with insufficient reverence. The issue in contention is a point of information, not a point of attitude.
As for at work discussions of religion, I think it is a judgement call. I’ve had really good discussions on religion at work. I was in an office once with a Unitarian and a Buddhist, and we had some very interesting talks.