Most Helpful Guy
It could be that he has things going on, but I'm not sure what else to tell you other than you need to set a time limit on this "break". Tell him that he has a week to sort out his feelings, you'll go off and do your thing - and at the end of that week he needs to decide whether you two are getting back together or splitting altogether.
Most Helpful Girl
Things most likely didn't change so fast, he was probably just hiding how he really felt. I've been with my boyfriend for 1.5 years and a couple of months ago he also acted weird and when I insisted on knowing what's up, he eventually said he wanted a break. Like you, I was taken by surprise because up until that day when I noticed he was weird everything seemed fine. But I know he's had these thoughts for a while now, I just didn't notice it, mainly because he never said anything.
Everyone told me that I need to give him the break that he wants. There were times when I wasn't sure if we're still even together or not, and like you, I wanted him to know that I care, that I want this relationship, that I want to work on things, etc.
A friend gave me really good advice. He said that if I really want him back, I need to give him the space he asked for. He said that I need to allow him to miss me. That means no contact. At first I felt anxious about this because I thought that if I don't contact him, he'll think that I am no longer interested or have given up on him or that he doesn't really mean all that much to me. My friend said "trust me, he knows you want the relationship". It took me a while to finally go through with the contact, but one day I just did it and it took less than a week for my boyfriend to snap out of the "I want a break" phase. Honestly, don't look for excuses to contact him, don't think you have to let him know how much you care. He already knows. Give him a chance to miss you. Good luck! Oh and yes, no ultimatums, they don't really work. Of course, should he still act like this in say five weeks then I'd say it's fine to ask him to make his mind up, but I wouldn't pressure him just yet.