That idea is called sola scriptura (the Bible alone). If everything that he believes is in the Bible then this must be in the Bible. Ask him to show you where it says that everything that is legitimately to be believed by a Christian is in the Bible. He can’t do it because it isn’t in there. He will try to do so by taking you to 2 Timothy 3:16 which says
“All scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness, so that the man of God may be thoroughly equipped for every good work.”
And, of course, this is true. But that doesn’t say that ONLY scripture is useful for that. Notice that it doesn’t say “scripture alone” or “scripture but not tradition”. Look at it this way: I could legitimately tell someone that learning Math in school is useful for their education. This is true. But that doesn’t mean that English isn’t important, too.
He may try to say only the Bible is to be used by Christians by quoting verses where the Bible speaks negatively about tradition. Search this site for “sola scriptura” and you’ll find more to help with this viewpoint.
He explained to me that there are a set of core Christian beliefs that renders you a Christian or not. All the other stuff is minor.
Like Roman Catholic said, make him show you where that list is given in the Bible. Protestants have different lists of the “core beliefs” and the “optional” / “non-core beliefs”. These lists are always just the opinion of the one making the list. Nowhere does the Bible give such a list. Press him and make him show you the list.
While you’re at it, ask him where the Bible says which books are supposed to be in the Bible (the canon of scripture). Surely this would be the most fundamental question of all and surely (if he is right) then the Bible would list the books - right? Nope, it doesn’t. The table of contents was not part of the original inspired books. So, for example, who says that James is part of the Bible? Martin Luther didn’t think it should be while others thought it was. Again (as with almost everything Protestant) it’s just his opinion as to which books are in the Bible.
How do I begin to talk with him when we meet? (against Fr. Serpa’s advice, of course, as heard on Catholic Answers Live a week or so ago.)
Do we talk about Peter’s Primacy, the Sacraments, Matthew 16, the Eucharist?
I’m not very good at evangelism so can’t help much. But I can say that you’ll not get very far (at least at first) by discussing Peter’s primacy, etc. I’d start by casting doubt on his ideas that everything is in the Bible and the Bible is our “authority”. Do so by asking where the list of “core” beliefs is that one must believe to be a Christian. Ask how he even knows that those set of books really is the Bible and that others are not allowed.
Also, sola scriptura holds that if an individual christian prays and seeks God about doctrinal questions that God WILL personally show that person the truth. Ask him how it is that Protestants disagree on almost every conceivable doctrinal question – and all of them claiming to be using this method.
Q: Why did Fr. Serpa’s radio broadcast tell you not to engage in discussion with this guy? Just curious.