Unfortunately there are people out there who struggle with autism and they dont even know they have autism.

First, I would message him and ask him, "please get in contact with a psychologist so they can help you through this struggle of letting go of me".

Second, clearly message him saying, "please do not contact me".

Third, if he continues to contact u, do not respond no matter what he says!!!!! If you respond to anything he says after telling him to not contact you; you will be seem as unstable and the police or court will not listen to you.
You will need to go to the police, which they will file a no contact restraining order or they will keep your pictures/ print out of what he has messaged you about and keep it on file for later use. Which doesnt sound very helpful at that particular moment, but in the long run can be helpful.