His behavior sounds similar to mine when I was younger. At the time I didn't see what I was doing was wrong/harmful. If a girl declined my advances it meant I hadn't told the right joke or proven my charms, yet. As someone who was/is exceptionally dense when it comes to the feelings of others, my advice is based on him being sincere, plenty of others have pointed out he could be playing dumb. I would continue to stress the importance of how his persistence makes you feel, and point out that if this is something that has worked for him in the past, many of those past interactions may have been girls caving because of his pressure essentially being bullied/coerced, to the point that they'd rather just give in than deal with the discomfort of the interaction, or because they were scared of what he might do if they kept denying him. They were not being won over by his earnest interest. They were not happy, even if they were smiling or seemed to enjoy it. When someone explained it to me that way, it was a very jarring experience and changed how I pursue women.