More likely a regular patron who was recently dumped by his lover.
I think you are right. The Colorado Sun has a few photos of him with his arm around another guy (same guy in both photos) and in one they are both wearing the rainbow headbands. It seems clear that the boy is gay himself.
https://www.the-sun.com/news/6722163/club-q-colorado-shooting-suspect-anderson-lee-aldrich/