I'm going to schedule a trip during my so's family reunion
Why do you want to schedule a trip during his reunion?
I know he doesn't like my presence during this event .
It is good to not want to be somewhere you are not wanted but it is not good to not want to be there where your future in-laws will be. It means the relationship probably won't last and you are wasting your time.
Maybe I will go, maybe I won't.
I hope you do go. It will be a good opportunity to get to know your future in-laws. Making other plans would not be acceptable.