My best friend admitted that he has feelings for me. We signed a lease on an apartment and were supposed to move in next week- now what?

I (24F) have known my friend (25m) since 3rd grade, although we didn't really speak until 9th. Over the last few years he's become my best friend. There's never been anything more than that.

He got a job almost 2 years ago and I got hired last year. Both of us are living with our parents and wanted to move out, but of course rent is expensive. We both needed roommates and he was incredibly specific about the kind of people he could tolerate living with, so we decided that we'd save up and rent a 2-bedroom apartment for a year or so when we we had enough.

Fast forward to today: he and I signed a lease on an apartment and are supposed to move in next week. I've been packing for weeks and I've been so thankful that everything was working out...except today, my best friend dropped a bomb on me. He said that during the last couple of months, and possibly longer, he's developed feelings for me. He's not in love with me, but they're there, and he asked if I would ever consider being more than just friends.

First, I haven't ever seen him as more than a friend. He's the guy that I could look atrocious around without feeling self-conscious at all. I'm the one that he can express his nerd rages to without judging him. And so forth. I can't bring myself to think about what would happen if we ever decided to date and it going horribly wrong. This is just a line that I don't want to cross. And while he's a great guy and more than capable of making a lucky woman happy, I've never been attracted to him. How do I help him without wrecking our friendship?

Second, what do we do about the apartment? I don't think that living with him is going to help his feelings fade, unless I act abominably.

I told him that I needed some time to think, but I really don't know where to go from here. Maybe I should have seen this coming, but looking back, there were never any warning signs. Never the slightest indication that after 10 years of being friends, he wants more.

TL;DR Best friend and I decided to be roommates last year after we saved enough money. Money has been saved, lease has been signed. Supposed to move in next week, but today my best friend told me that he has feelings for me and asked if I'd ever consider dating him. Don't know what to do.