First, I applaud his effort to respect his aunt's home and beliefs. Regrettably, being honorable is not valued as highly as it should be. Think of it this way: whomever this guy marries, her wishes/beliefs will be highly regarded by him.
You say you have a tense relationship due to his inflexibility. The only action you cite is his committment to respect his aunt's home rules -- the aunt who has provided shelter and a home for him while he pursued his MA.
You also say he was pursuing his MA and working. Both of these endeavors are taxing in and of themselves, much more when pursued concurrently. No wonder he wants to get some sleep -- i.e. "leave around midnight".
Is the tension only because he refuses to compromise his word to his aunt in order to please your sleep over request? Or is there more to the "tense relationship"?
You helped him with his work -- so does that obligate him to take you on a trip? Certainly, it would be nice if he wanted you to come - maybe viewing it as you both worked on his MA and both deserve a break... but maybe he wants some alone time. I am married and committed to my husband, who helps me everyday in multiple ways -- but there are times I just need to get away and be me.
Now, to address your question, "Why didn't he invite me?" I just read in a response that he said he hadn't seen his family in four years. Do you feel safe to take his answer at face value? If not, you guys really need to talk. You do not need to be in a relationship with someone you don't trust.
Maybe you feel slighted bc you haven't met his parents. How much discussion have you had about committment levels? Are you guys devoted to one another?
I hope you discuss your feelings with your boyfriend and are able to resolve the conflict.