As a starter, you could ask him:
- How many countries currently have laws making it illegal to be cishet, sometimes punishable by death.
- In the US, how many states have passed laws making cishet relationships illegal. What year were they repealed?
- How long did it take for an American president to openly support cishet marriage?
...but like other folks have talked about, it's difficult to use logic to get someone out of a position that they did not logic themselves into. You're arguing with feelings, and so long as he feels oppressed, that's going to be the truth of his world.