I'm not very good with any of this, but I have talked at length with NT friends about this (and quizzed them on all the confusing bits) and I conclude that when it comes to long-time friends and romance, NT's are just as clueless as us.
Both people involved are scared to embarrass themselves and lose the friend, and so one gives even subtler clues than usual, and then other isn't sure whether they're picking up the clues right or not, and so only reacts very subtly, and nothing happens (until perhaps alcohol intervenes).
Or perhaps one feels love, and so searches every interaction for clues of the other's love, and misinterprets out of pure hope.
Anyway, I have heard of many other scenarios, but the general message is that this is tricky for anyone, so I'd say no one could be sure, but you may as well try and see what happens, if you really like him.