Maybe you could write a letter. But it sounds like he is very resistant to the idea. Also, maybe don;t use the words 'need more' which could label him inadequate in his mind - rather maybe 'need variety' or 'appreciate many humans' uniqueness'.
maybe you could ask him if he ever feels attracted to other people and let him do lots of the talking, and then take it from there.
Maybe you could also state baldly: "You seem very resistant to talking about what i am trying to tell you. What exactly makes you hate the idea so much?" Expect a lot of false starts, i doubt he knows exactly what it is he hates.