He cannot fight (and cannot help defend the collapsed tunnel) until you resupply him: his blade is worthless without Sleeper's Fumes, of which he has run out, so you must bring him more before he will join the nest-clearing effort.
Old Mine, by the collapsed tunnel: A Brotherhood templar hunting minecrawlers in the Old Mine.
Gor Na Vid can be found in the Old Mine (Templar).
Gor Na Vid: May the Sleeper guide you.
You: I need your help. We found a way to clear the abandoned tunnel, but we need fighters to defend it for when the crawlers come out.
You: Why not?
Gor Na Vid: Because I have run out of Sleeper's Fumes, and my blade is worthless without them.
Gor Na Vid: Ahhhh ... yes. You have my blade. Where shall I go?
You: I'm looking for Lester.
Gor Na Vid: He's down by the collapsed tunnel.
You: I found the nest of the minecrawlers!
Gor Na Vid: Good.
Gor Na Vid: Thank you.