Older guy asked to add me on FB but won't reply?

[Jeff]

Try saying something more than just "Hey".

Strike up a conversation with him. "Hey" is a good start, but following it up with "haven't heard from you in a while and wanted to chat with you". or, "How's life been? What have you been up to?"

Anything but just, "Hey".

that makes it automatically sound like your encounter with him was just awkward through and through.

As far as the amount of FB friends, it might mean he just adds everyone he meets, but I wouldn't say he would want to add you if you weren't the least bit interesting.

Build that connection with him. Just don't expect it to happen overnight.