I'm in his camp of relationships shouldn't be hard work. I wouldn't take kindly to someone pressuring me to change my view. If I thought polyamorous relationships were going to be harder work for me than monogamous ones then i would be monogamous. Your partner obviously feels that further opening of his relationship would be harder work for him and is therefore apprehensive about it. I support his apprehension entirely if he feels that a polyamorous relationship would be too much hard work to adjust to and be happy.
Please forward any comments, complaints or suggestions about my various thoughts and opinions here