How can I convince my parents to let my boyfriend sleep over?

There's a very good chance that you won't be able to convince your parents at all. But if it were to happen, you would pretty much have to be an adult, have a sit down with them, explain the situation, and ask "What would it take for you to let bf stay here next summer for a few weeks?" If I were a father, I'm not too sure I would want him in a hotel, because I would be worried that you might end up there too. If they are immovable, you can ask them for suggestions. Perhaps you have a male friend in your area that would be willing to put him up. Perhaps between everyone involved, you could pitch in and afford a hotel. Some cities have hostels that aren't too expensive. In any case, your best bet to be treated like an adult is to act like an adult, and you do that by keeping everything out in the open and on the up & up.