Hun you ain't got nothing to feel guilty about, feelings can't be forced or changed you didn't feel the same and that's okay, he probably decided to confess when you got a gf so that he could hear you say no and move on, you didn't brake the friendship he did because he needed space to accept it, it might be that he thought he could get over it faster after hearing the no but it didn't go as he planned. I know it's difficult but you should stop trying to contact him if and when he'll feel ready to be friends again he will write to you, it's worst if you keep reaching out especially because he might not be over you still and that would also be a problem for your relationship cause he would probably not be very respectful about it.
It was with my best friend. He confessed to me but I don't feel him that way. So I rejected him but he said we couldn't be the same any more and went silenced. I still trying to contact him to these days but I never heard from him again. I still feel guilty or feel like I broke this friendship.
(FYI: that time, I have my girlfriend and we are still going strong for 3 years)