If someone with PTSD (my SO) commits to getting treatment, staying on meds, etc. and doesn't do these things, what are the appropriate "consequences"? I want to have healthy boundaries for both of us, but I don't understand how to do that without making him feel like I'm imposing my will on him - a major trigger.
Healthy boundaries have little to do with "consequences." They are not about trying to penalize someone out of problematic behavior or choices. In fact, they are not about changing or controlling the other person at all. Ultimatums tend to fail and turn into power struggles, at best.
Healthy boundaries are about our own limits, and managing what we choose to do and let into our lives and not, in response to another's choices.
You made it clear at the get go that you would not be in the relationship unless he was in treatment. He chose not to be in treatment for a time. It's good he is scheduled to get back into treatment. I would take time to sort out what you need to stay in the relationship. If you need him in regular treatment in order for you to stay in the relationship, then it's reasonable to communicate what you will do to take care of you, and manage that limit, if he chooses to not go to treatment. Something like, "if you make the choice to not go to treatment, in order to take care of me and my needs, I will need to leave the relationship." It could be that you find another place to stay, that you break up for a time until he is willing to go back, etc. Whatever the choice is, it has to be about you managing what you let into your life and not, and it has to be something you do. It can't be, "if you don't go to treatment you need to...."
I would let him and a professional sort out what that treatment will be, but it's ok to make it clear you can't stay in the relationship if he doesn't get it.
If you are not willing to keep that boundary, that's ok - but I wouldn't bother continuing to express that he needs to be in treatment for you to be in the relationship with him. That will just turn into nagging and trying to change him, and sending him a mixed message that your boundaries are not really boundaries, there is no negative outcome if he crosses them. If you are not willing to leave the relationship if he doesn't follow through on treatment, that's really ok too, but don't hold that to him. What he is responsible for is his choices. It's your choice to stay or go. If you stay, other possible boundaries could be that you spend less time with him until he gets treatment and you'll instead spend more time with friends or etc. If you make it clear you are doing this to take care of you and stay within the limits of what you can do, that's less likely to be something that brings up his defenses. It's about you. And what you can and can't do. Rather than him being at fault or to blame.