Same thing here - I was attempting to romance him, and ended up not having any dialogue options with him for the longest time.
Figured they'd open up as soon as his romance triggered - but just found out that him and Dorian are in a relationship. Somehow I got locked out of a romance with him, even though I flirted wih him all game and did his personal quest. :\ Really annoying because I'm 77 hours in... but if it weren't for the fact that I absolutely love how my F!Elf Inquisitor turned out in the Character Creator I would start a new game in a heartbeat. Really just want to romance Bull. :\