Ultimatums are hard. You need to be very sure what you are doing. I've given a couple ultimatums but have not followed through, basically because I was using it as a tactic to manipulate his behaviour. I'm trying to learn a new way of dealing with his addiction where I can positively reinforce his good non addict behaviours, which is more my style.
If you were to give an ultimatum, are you prepared to follow thru with it?