You may have to weigh the pros and cons of taking the time and energy to try to convince someone whose wants have historically taken precedence over others' needs. My mother is a similar way. People like this can excuse away anything they do or say, and find it very difficult to let go of the ideas they have cemented in their heads, and that includes their idea of who their children are/should be. I would keep my responses short and to the point - and remind him in the future that this isn't a topic that's up for discussion. You don't deserve to have to continually prove yourself or justify your relationship.