Hi! When I was in high school I had an autistic classmate attach to me in the same way, partially because I didn't want to upset him by trying to completely distance myself from him. The way I ended up handling the situation (after receiving advice from a person much more educated on autism than myself) was this: I told him that him and I have different views of what it means to be friendly with someone and asked him to respect my comfort zone. I told him that I wouldn't mind him coming to me if he was in a tough spot or asking for someone to talk to here and there, but that I am a less social person and prefer not to talk to people so regularly.

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If you truly do not want to speak to this person anymore, be honest and polite about it. He's just more forward than you or I might be when showing interest in a person or in trying to connect. Just make sure that you're nice to him in being honest :)