Np
I try to avoid arguments based on the Greek unless something is extremely clear and unequivocal. This is especially true with Calvinism, absolutely ever verse of the Bible has been commented on (though all of them not very well, imo) from an exegetical perspective. Besides, there's always someone who knows it better than you, so if it's your primary weapon, it won't be long before you are disarmed and totally helpless, lol.
You probably aren't going to get this guy to change his mind. If he's got a good basis of Greek, then he's been studying for a while. And if he's been studying for awhile, his conclusions are probably set in stone. It may be useful to talk to him once or twice just to find out where he is on the issues, but "conversion" isn't all that likely. Imagine if he tried to get you into his camp! However, with that said, I do think it is extremely important to not let clearly Calvinistic statements go unquestioned. You may not change his mind, but you can make sure those he is teaching are getting both sides of the equation.
For example, suppose he is teaching and he makes a comment in reference to final perseverance of the saints. I don't know your view on that one, but I personally reject it. So, he says something about "real" Christians persevering until the end. That opens a good door for you. You ask may ask him if he sees a difference in preservation and perseverance. You force him to admit that the latter is dependent on the former, and that it is all God's work. After all, if a person must persevere in his faith by his own power, then clearly he believes in a works based salvation! No Calvinist believes that, so he clarifies that it is God who preserves our faithfulness. Fair enough, so then you may ask him what that does to the doctrine of assurance? Can we really know we are saved? After all, aren't there people then who think they have faith, but they really don't? They are continuing in their own power, and eventually they will fail, because God is not preserving them? This must be true, so we see that God only preserves the faith of His elect. So, you can ask the all important question: How can you really know that you are elect?
At this point you'll get some answers about the fruit of a persons life and the witness of the Spirit in you, blah, blah, blah. Now you can wrap up by, instead of arguing and starting a debate, simply saying something to the effect of, "Well, I see what you are saying. But, somehow, the idea that we have to look to our works to know if we are saved doesn't fit with the whole of Scripture to me. Jesus said that he who believes
has everlasting life. John talked about knowing you are saved. It sounds to me like you really can't know, but that you can only be 'pretty sure.'"
He'll offer some weak defense, and you let it drop. Now, what has happened in this exchange is those in the room are seeing something very, very important. They are seeing the very basis of Calvinism. You have exposed what you perceive to be a weakness. You aren't arguing or making accusations. You are asking questions, and the other people in the room may stop and think, "Now, wait . . . this doesn't make as much sense as I thought it did!" All this, and when you go back to the original line of thought, when he picks back up with teaching a Scripture from a perseverance perspective, the audience will be aware that he is teaching from a certain bais that may or may not be right.
I wouldn't do this all the time, but when appropriate. I do it in my classes at school. So yeah . . . just keep studying, and listen closely to what he says, and ask him questions that reveal the presuppositions of his position. Then ask a question or two about the implications of that presupposition, and you've done the class a huge service by exposing them to both sides of the discussion.
God bless