Forced to cancel our first vacation??!!

Lillian
Trying not to cry in anger as I write this. Here we go. I'm graduating college in December, and as a graduation gift, my parents invited me and my SO to go to florida with them (they were planning on going for my mother's golden birthday). This is the first time they have allowed me to invite someone to come with us, and I was very excited to share this experience with my SO. the plan was to go to Florida, Disney world, universal studios, the whole 9 yards. I'm almost positive this will be the last family vacation since I'll be graduated and my parents aren't getting any younger.. 😞 
So the trip is in January, at the beginning of my SO's first semester of grad school. He emailed one of his professors and said he was asked to go on a family vacation and literally asked permission if he was allowed to go on this FAMILY vacation. He made it seem like it was optional and no big deal. So of course the professor says it would be "detrimental" if he missed a week of class (the class is 1 hour 3 times a week). Seriously? 3 hours of sitting in class going over a syllabus and learning names or making lifelong memories with loved ones? If it were nearing the end of the semester I would understand but it's literally right at the beginning. So now my SO has decided he can no longer go on this vacation with me or my family. Yet you can bet he'll be at the bars and getting drunk with his buddies when I'm gone. How can I not be furious at him? I understand that school is important but he could have worked around it! I don't know what to do and I can't get over how upset I am. Now I have to break the news to my parents without sobbing. Any advice or input is appreciated but please don't be rude. This was really important to me.