It'll change your relationship a little bit, sure. You'll have to get used to cohabiting and melding your possessions and personal quirks. But remember that it's only temporary. You get a taste of living together without actually having to take the plunge. Your guy knows he'll be moving out once the summer is over, so some of the pressure of moving in together is taken off.

First off, set aside some personal space for him. Give him a bookshelf, a couple drawers, etc. If he has comic books, video games, marathon medals, whatever, give him some space so he feels at home.

For the rest of the space, maybe decorate an area together or pick out a new piece of furniture with your other roommates. Include your guy on apartment decisions so he feels like he's an equal part of home and not just your boyfriend who is staying there.

Otherwise, continue things as they've been. If there were days when you did your own thing and only saw each other briefly, continue that. Keep having "date nights" and go out and do fun things together. Don't settle into a domestic rut. Obviously you'll be spending more time together, but the relationship doesn't have to change if you aren't ready. There's no reason to force yourself into domesticity if this is just a temporary situation. Maybe it's a test to see how it'll be living together, or maybe it's just a few months where you guys played house. It's more important in the long run to focus on making the relationship work and to give each other space when needed.