He gets captured in the underdark and is in the grymforge after act 1. You have to rescue him again down there before he shows up at last light inn. I just ran into this on my current playthrough.
I skipped the underdark at first and rescued moonrise prisoners. Barcus wasn't at the inn and wulbren was doing the same thing mentioned above and not talking to me. I went to the underdark and rescued barcus again. There's no option to tell any of the gnomes that I already saved wulbren from moonrise in those conversations.
When I play again I'll have to check the inn to see if I can continue the quest.
