Say I wanted to learn with an elder fellow (80s) who's always Been irreligious his entire life. What should I learn with him to bring him to Religion. Keep in mind: He can read Hebrew, he believes in GD & his main problem is that the Rabbis came along of enacted their own stuff. He essentially only believes in the Written Torah. None of his immediate family is religious either. What does traditional kiruv have to say about this, what sefer do I learn with him; Tanach, Mishna, Gemara, Mussar?
Rabbi Aryeh Kaplan has numerous book geared toward Kiruv. One of them has something to say about the Oral law. See "The Handbook of Jewish Thought" (Vol. 1). This author had a gift in explaining torah topics in all categories.
As far as learning torah with the student, Pirkey Avos is very popular in kiruv as it is an easy read and you can use it as a "spring board" to launch into various topics that interest him and hit home.
As for learning gemarah, if the person likes to complete something, making a goal to complete gemara taanis is as easy as they come and is one of the shortest gemaras in shas.
If the person is an intellectual, the second perek of bava metziah אלו מציאות is a good beginners gemara as the gemara is a fairly easy read but it has loads of lumdus to dive into. Also the topic is easy to relate to for someone new vs. "my ox gored your ox" or "how does one acquire a woman (to marry her)"
A good option might be Horeb, by R' Shamshon Raphael Hirsch. It's been translated to English from the original German by Dayan Isidor Grunfeld. He breaks down the rationality behind the commandments, focusing on the relationship between G-d and the people. He also uses a highly textual approach. It will introduce him to Orthodox Judaism and hopefully break down the walls that he has against the Rabbis without getting "bogged down" in the traditional style of learning that is Mishnah/Gemara.
It's important in Kiruv to not fall into the trap of "He's at x level, so let me learn y with him." You need to learn something that will address his pain points, rather than focusing on the fact that he could ostensibly start to learn Mishnah/Gemara.
Where did you meet him?What is he looking for and why is he interested in learning Torah with you?
It is difficult to answer you without knowing the answers to those questions.
I'm no Kiruv professional but I do learn weekly with a secular Jew. He doesn't keep Shabbos but he does seem to believe in Torah S'Baal Peh. I only learn Gemara Seder Nezikin with him and it goes very well.
If you are dealing with someone who thinks the Rabbis came along and enacted their own stuff perhaps you should start with Bava Kama, Perek HaChoval.That perek would give plenty of opportunities to explain the mesorah of Torah S'Baal Peh, rabbinic authority and related topics.
Caveat: Before teaching this topic, make sure that you really are knowledgeable and understanding of it