As this was a 'tender' spot Fanny uses to sway him irrevocably as JA tells us that he found that argument 'irresistable'. I think he secretly resented his father's marriage for years. Is it possible that Henry Dashwood had to marry John's mother for her money, but married the current Mrs Dashwood for love? I have nothing but conjecture, but we do know that Henry's first wife had quite a large fortune - and Mrs Dashwood by contrast, had nothing. He may have noticed that his father loved his stepmother more than he loved his own mother.