first of all dictating a life would be outright telling him that he cannot do what he wants to do. YOu have not done so, you've been clear that this is not something you want in a relationship and that's fair. So if him wanting to be a way 2 to 3 weeks a month on a job is what he wants, his take is that you're supposed to mold your expectations to what he wants and that's ok, but you're not allowed to have your own expectations based on your values or needs? Now really, who would be dictating anyones life if that's the case? think about that.

Secondly, as you've stated this is not new information to him. It's not that just at this moment he was made aware that you can't do the 1x a month being together thing and you've said that's what you've said from the start. He has no basis for his acting shocked about this but once again, it's expecting you to have changed your views and values since the beginning til now. Now that he wants that it's supposed to be ok with you - again, this is HIM expecting you to mold in a way to fit HIS desires.

A loving relationship and a loving person looks out not just for themselves but considers the partner's needs too, sometimes even before their own. This is not happening in your relationship.

Who in a commited and strong relatship has a stipulation that they get a job where they require it to keep them away for 2 to 3 weeks out of the month? I'm sorry but for me if I am commited and in a relationship with someone I want to be able to be there at all costs and I work around that premise. this sounds even more like he's not a good fit for you.

Even without him taking a job that takes him away for much of the time, I'd say you should continue with the consideration of ending this relationship. He's not at all committed to making it work nor is he thinking of your needs, values and expectations.