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If you really have your grandfather playing cRPG for you, then maybe you should be mad at yourself. My parents are close to 70 years old too and they have never played video-games, and, if they did, they would probably do it like him - apart from leeching, it's clear that video-games aren't his thing.
So, if your grandfather is really playing for you and doing these things, maybe, rather than getting angry with him, you should stop being so greedy and let him do stuff an old person would really enjoy, rather than grind xp for you.
If it's really true, what I've seen is your character leech most of the time, then move a couple of feet, leech again, then move again, fail some attacks then either die or go back to leeching, and my impression is that he doesn't grind for you because he really likes the game.
So, if you can only play on the weekends, then too bad. But don't exploit your old grandpa for your greedy desires for a stupid video-game. I'm sure he has more important stuff to do and there must be more productive ways of pleasing your grandfather's love to his grandson.
If I'm wrong, and he really enjoys the game, then tell him to go spectator or leave the server whenever he has to go AFK.