Is it necessary to inform an elderly grandfather about the ruling on letting one's garment hang below the ankles (isbal), and how can this be done without embarrassing him, especially given that the questioner is 16 years old?

The disobedient father and those in a similar position should be advised gently, kindly, and wisely. One can seek help from righteous people or beneficial tapes. Among the things that can help him abandon the practice of isbal is to buy him clothes that are not musbil (hanging below the ankles).