First of all, I'm so sorry that you're going through this, it must be very frightening and I hope your new medication is effective.
Re. The trip. Your DP was unreasonable to announce a long holiday like that, but I have the impression that they were all talking about it at work and he was pulled into the excitement and rushed home to tell you all about this once-in-a-lifetime opportunity. He was fired up and not really thinking it through - like an excited child!
In the cold light of day, there's a lot to consider and he's probably feeling deflated...again, like a disappointed child! I think you might be the more mature person in your relationship. 
Anyway, as PPs have said, 2020 is a long time in the future and with careful planning, you might be able to make this work. I'd talk to him again and say that it's a good thing you've got plenty of notice, as there's a lot to sort out. He can't just swan off, he's got to help you figure out how to make a 13-night trip work!
Make a list of what needs to be done (e.g. someone to be with you every night) and work WITH him to figure out who can help.
I've noticed that my DH is much more responsive when there's a plan to address a problem. He recently went on a long trip with DD and we worked together to make sure things were running smoothly at home.