Mylovelygreendress
It does seem weird.
I've tended to muck about a bit on this thread because I found it impossible to take it seriously but if I were to engage...
There isn't a question to be answered - the OP says "his expression is open to many interpretations".
Yes, it is. That's because we're all human, not emojis who 😁 when happy, 😪when sad and - if some posters here are to be believed - 😂😂😂, to the point of dehydration from the tears of laughter, every time they find something mildly amusing.
However, maybe they're inviting other people's opinions. So...
I have got quite a few similar photos comprising my DH, a friend of mine and me. Friend and I are in full conversational flow and DH has let the mask of being interested in whatever we're saying slip and is thinking about something else.
Conversely, there are a few snaps of DH, a friend of his and me in which I'm looking a bit abstracted because I've lost the thread of the subject (chess openings, say*) they were discussing.
Both of us would have probably been looking fairly animated just before and just after that split second.
And that is what I think is happening here.
*Not joking. It's surprising there aren't more photos of me looking a bit bored