May I suggest you just keep him hanging?
OK, so you were a bit worried about the school reunion. First person you meet, wow, wants to date you.
Now I'd say that is about a good an endorcement as you could expect. (My heart lept a bit when I read it. )
You could ask him to escort you. You could simply met him there. Or you could simply go and see how things pan out.
If this were me, I wouldn't agree, at this point anyway, to become too involved. Simply because you haven't met each other for a long time. You have certainly taken different paths. You have a history, however positive, it is a history.
I don't say this simply because of your change in gender. I say this because, after a time, you are different people.
I can understand his perspective. But this is a school reunion, not a reunion of two old friends. I should think there's gonna be a lot of people there, male and female who are going to be interested in you. Some may be a bit negative, but most will be positive.
Now the hard bit. How to turn down a guy without hurting his feelings?