Sounds like the correct diagnosis, Old Hand products do a charcoal tablet that really works. I had a Van Reet cock that was on deaths door, rocking on his feet. A week of isolation on a course of these and it cured him and he's repaid me over and over again.
If you gently squeeze the pigeons crop, you should get a foul smelling bile type water come from it. After doing this, get it on the charcoal tablets.