How do I tell someone my friendgroup and I don't wanna hang out with him

whether we're in a group or individually roaming the halls at school he always comes up to us to talk and hang out, as friends do. The problem is that none of us want to be around him but we don't know how to tell him that.

The biggest problem we have is that he doesn't read the room and doesn't pay attention to anything. For example we were eating lunch and talked about a philosophy essay my friend did about the morality of lying. He sat with us during the whole conversation and out of nowhere said "What? What do you mean lying in *insert situation* is okay?" when in fact it we were pondering whether it is or isn't moral and we had to recap the whole discussion we had had.

Another time he came up to my two friends and asked what they were talking about. One of them said that it was a topic that she only wanted to discuss with the other friend. He then proceeded to ask *three more times* what they were talking about, she gave the same answer every time and he just wouldn't take the hint.

Of course it's humane to mishear and misunderstand things but this happens every single time we talk and it's tiring. Often we will ignore him and give him very short answers because we just don't wanna talk to him and that makes us feel like such assholes but we just don't know how to politely tell him that we don't enjoy him company.