Well if it's any consolation, you did the right thing by not responding. It's only been a short time so give yourself a bit more time to adjust to not having him in your life. However; don't stagnate there wallowing in your misery. It's now time that will heal you but rather what you do with your time. Get back out there and do the things you like to do, consciously change any thought of him that pops into your head to something else, stop going online for a while or, block him if you're tempted by his attempts to reach out to you. He lost feelings for you so why even be his fking friend? Surely you're not wanting to keep him around when he's demoted you from gf to just friend?
You're young, chances are very high that he'd not have been your LIFE-mate. Accept that and take charge of your getting over him. Time to start on the road to being a happy single so that you can find someone close to you who won't back out of the deal so easily.