It could be one of three things.
1] You're right, he just doesn't get it yet. With time he'll learn.
2] He doesn't understand that the person isn't interested until the point at which the person walks away. By then it's too late, so why bother worrying about it? He knows something has gone wrong, but it's difficult for him to decode what just happened. He can't correct his approach unless he knows exactly what he is correcting, so he shrugs it off and tries again on someone else.
3] Perhaps it does bother him a little, but it's not worth having a big reaction to. But again, he doesn't know any other way to connect to people, so that's what he does. My mom knows me better than anyone else does, but even she can't always tell when I'm bothered. Most of the time I just don't react vocally, facially...ect. I may tell her later how bothered I was here, anxious there, and usually that surprises her quite a bit, having observed my face and manner at the time as unconcerned.
There could be others still, or a combination of any of those. Mabe you can tactfully help him to look for those cues, explaining to him that some people may not want a long conversation, and some people may not want to talk about the things he wants to talk about. You can even try to roleplay with him to help him listen better. He may not get the cues right for a while, especially since everyone has their own individual ones (my favorite excuse), but it will at least give him something specific to practice on instead of just guessing how to do it.