How do I let this guy know I don't like him?

There's this guy in two of my classes, and were both 16. He used to be part of the "popular" crowd but after going to rehab for the summer he stopped talking to his old friends because he was clean and wanted to start with a new group. All of his old friends say that he's changed from before, and before I thought they were talking about the fact that he doesn't use drugs, but now I can see why they are saying that. When you talk to him, it's like he's in a different world. Your saying something to him but you can tell it doesn't register with him and he's really slow with everything. Well anyways he always sits beside me and stares at me, and everyone says he likes me. I'm always nice to him and try to start conversations, but we have NOTHING in common and every time we try to start a conversation I feel like he will get confused because he always does. It usually ends up with me talking a lot about who knows what but I feel like he thinks that i'm nice to him because I like him. He always asks me weird and personal questions, and I don't think he realizes. (i.e what did you dream about last night or what brand of jeans do you buy) and he says it in a really unattached way.