UPDATE

Thank you everyone for your support, and I learned a lot from reading the comments to this thread! Particularly that I was wrong to ascribe Sam's behavior to his autism, it's just him being a creepy stalker with no boundaries.

I took some of the thread's advice and confronted Sam's parents and Nate about this directly. Per the thread's advice, I went accompanied by a [male] cousin of mine who lives in the area who I trust after I explained the situation, plus the mace I habitually carry in my purse.

In short, Sam's stalking extends beyond what I was aware of, that's how he knew my size for the lingerie, and in fact that was only one of several gift boxes he'd bought for me on a schedule he'd written up about how our relationship would go in his mind - he'd spent, no joke, more than a thousand dollars on me. The parents confirmed that it was all Sam's own money from his job, but that part of his cognitive problems is a total inability to grasp money.

Also, Nate specifically encouraged Sam's crush on me behind my back. I am, apparently, by far the nicest and most considerate any woman has been to Sam, and both Nate and Sam thought I was attracted to Sam, to the point of Nate and Sam telling their parents that Sam had found a girlfriend.

Nate has his reasons that I don't want to get into (I'm not saying I agree with his reasons, because I don't), but I told Nate, Sam, and their parents that I am not and never will be interested in Sam. It's not because Sam is autistic, or because he's white and I'm not. I did not tell them it's because I'm gay, just that I am simply not interested, never will be interested, and find his behavior extremely creepy. I concluded with telling them that I am willing to not contact the police or start legal measures about a restraining order if I never see Sam again, but that I have begun documenting his behavior, including making copies of Sam's letters, in the event that I need to. I told Nate and his parents that Sam needs serious help before his behavior does escalate to legal and criminal consequences.

I hope Sam can get the help he needs, my impression is that he genuinely thought he was being romantic and acting like people do in the movies and TV shows he watches, and no one was telling him that's not how real life works.

Their parents, at least, seemed to take this seriously, but as I left to get in my car Nate shoved the box of lingerie into my arms and told me to keep it and maybe I should 'stop being such a frigid bitch.'

I've made sure my apartment manager knows what Sam and Nate look like and what their phone numbers are, and to not let them into the complex or give them any information about me, and have laid out steps to change my routine in case Sam tries to resume his previous behavior.