I suspect my neighbor has some mental problems, how would I go about isting him without offending him?

I live in an apartment complex. At least 4 days out of the week one of my neighbor comes outside cursing loudly in an argumentative tone. He talks as if he is arguing with someone, but no one is there. He once asked to used my computer and once he finished I saw that he had been using search engines to see if the FBI or local law enforcement had any files on him. It's beginning to become a problem because my other neighbors are complaining and the police have been called as well. Earlier today he was cursing again and I checked to see what was wrong and he said that police detectives put a probe of some sort into his head. He's been my neighbor for over a year and he wasn't like this when I first moved in. I know he smokes pot regularly as well, something I've talked to him about stopping due to some of his behavior. What can I do to help?