I used to do that when I started my job, I had to or I would have a panic attack. If people talked to me I would talk back, but never initiate conversation.
It was nearly a year before I could force myself to greet everyone each morning and make small talk. And I still feel panicky doing it.
Maybe he has SA too or doesn't like to talk to people much. I know a person like this, he's not shy at all but he just likes to be on his own. You said you did try to engage him in conversations. In my opinion if he doesn't seem interested it might not be a great idea to try talking to him, I could be wrong though.....
To me it sounds highly likely that he has SAD, but I wish people wouldn't be so definite about it. While he may sound like us, there are people who appear to have SA but in reality feel they're "too good" to talk to people or just don't feel any desire to appear friendly or socialise, either due to depression or some other mental disorder or just plain introversion.
Also sounds like me at work haha however after a few convos with people (that they've started) I will at least try to say hi but you don't know what's going through his head. I find it quite amusing that some people on here can be so unsupportive about other peoples problems
Its always interesting to see examples on here where the roles are reversed. Reminds me a bit of a retail job I worked awhile back. I made a bit more of an effort to be social though. He may be avoiding talking with people to avoid feeling rejected. Doesn't seem to be taking into account that at the same time, he is automatically rejecting those around him. It may take a little bit of emotion/annoyance to get your point across. Next time he tries to blow you off, try saying "Come on, I'm making an effort here". Been in situations before where my anxiety has prompted me to ignore the feelings of others and how my behavior affects them. Try adding a little emotion to your conversation if you feel safe about it. Just a thought from an introvert that has used the same kinds of defenses you are describing. People have tried it on me, and it helped to make more of a connection.