I'm going to talk to this homophobe on the bus and act like I agree with him to be civil and stay in contact with him.
Could you be more specific about why you would want to agree with him, even if it's just an act?
I know he's a homophobe but I decided to talk to him because he's sitting alone and his eyes seem sad
You seem like a good person as you feel empathy for this person, but letting them think that you support homophobia isn't a good thing to do. It's unfortunate they seem sad, but you probably should not engage with them if you want to avoid conflict.
What if he's just misunderstood? We all get to that point that way at times.
I'm sure there are aspects of their life where they are misunderstood, however that doesn't excuse them being hateful and discriminatory towards others. I definitely think ignoring them is the best plan of action here.