While you are technically an adult, you are living in your parents’ house for the summer. This puts you in a weird place of being quasi-adult, quasi-child. Your father is used to seeing you as a child, and may be in active denial about you growing up and doing adult things (including things you don’t do, like drink and party). He might be imagining the worst, and hoping that he can keep you “good” for as long as possible. All kinds of things can happen at midnight on a Friday – he doesn’t want to imagine you doing any of them. Eventually, he will have to accept that you are growing up.
At the same time, while being a legal adult, you are not yet a full adult as accepted by society. You are still in college, which (especially if your family is paying for all or part of it) is an extension of childhood, a grey area. You have chosen to live under his roof for the summer, something adults generally do not do except under duress (unemployment, college, no money). Because you are in this grey area, you don’t yet have full adult rights – or responsibilities. You still have to accept the parameters set out for you right now.
The way I see it, you have a choice, since you’re sort of in between two worlds. You can step into one, or the other. If you choose to seek out the comforts of home, a father who would very likely come and get you if you called at midnight needing help, a (free?) place to stay and (free?) food and utilities and probably some cash over and above whatever your scholarship covers, these are bonuses (even though they don’t seem like it now) that come with the responsibility of obeying your father’s rules. If, over time, you decide to forgo these comforts, and instead provide for your own self, living in an apartment or house that you pay for with your own money (maybe sharing with roommates to make the finances easier), pay for your own food and utilities and any spending cash that your scholarship doesn’t cover, to take responsibility for your own actions (like not counting on your family for emergencies, either being mature and ‘adult’ enough to cover them on your own or develop a network of other responsible adults to lean on if need be), then you will have taken upon yourself the responsibilities of adulthood and with that come the rights of being an adult, such as coming home whenever you darn well please. ;)
Setting adulthood at 18 (or 19, or 21) is an arbitrary number. What really confers adulthood is the acceptance of adult responsibility. That may not be an option for you yet, in which case, well, it sounds like your curfew is 10pm if he won’t negotiate. (It is, however, his responsibility, if he’s going to be a good father, to let you grow into an adult role at the pace he thinks you can handle it. This is no guarantee that he will, just that he should.) It looks like the choice is up to you, now.
It is worth remembering that no matter how good adulthood looks, and how much you want a curfew of your own choosing, once you leave that childhood status, you can never really go back. In years to come, you may well actually miss being able to crash at home, nom from the parents’ refrigerator, and have someone worried about you all the time. The outside world is sometimes cold and bereft of comforts, and the joy of setting your own curfew will pale with time, especially if there isn’t anyone waiting for you to get home anyway. Choose wisely. :)