I liked this story a lot. One of my favorite ever, in fact.
I found the overwhelming majority of Justin's Aspie behavior very convincing. He does not handle the unexpected at all well, misses the obvious, and sometimes over-analyzes. That's spot-on, as is the fidgeting.
He's very in-tune with himself, and knows his limitations. That's unusual to that degree for an Aspie (or anyone) at that age, but by no means unlikely.
His ability to learn some things at freakish speed is not uncommon with Aspies, nor is some of the on-again off-again aspect of his social skills. In real life, it's more situational, and the story mostly rings very true in this regard IMHO.
Aspergers can be described in part as having the ability to sympathize, but lacking the ability to empathize. Justin, as we saw when Rob's dog died, can sympathize, but empathy is beyond hi, hence his trouble dealing with social situations, especially unexpected ones.
Justin's demand that anyone he dates be "out" makes perfect sense: he isn't able to keep secrets, and knows it. If he dated anyone who wasn't out, he would eventually slip, so this is the best way for him to handle it.
One thing I'd like to comment specifically on: the lack of detailed physical descriptions of the main characters, particularly Justin: I loved it!! The reason is that everyone has different tastes regarding what they consider "hot" appearance. So, I think it was left just vague enough, though with plenty of clues, that the reader can use their own preferred image.
As for the ending, I usually don't like unhappy endings, but this one IMHO worked great. All too many times I've read through 40 chapters of a story, then the characters finally get together, only to see one of them die in a fiery crash. Yankee definitely breaks a lot of molds, and this was one of them.
There are a few things I would have liked to see more of, such as the relationship with Dan, and Justin's home life (Two aspie parents? THAT would get interesting!)
I really liked this story, and am very much looking forward to the sequel, or whatever Dan might choose to write.