Why is he acting this way?

Boys are boys. They're interested one second and the next they aren't. Honestly, he probably didn't lose the attraction, but he lost the interest. They have girl ADD. They don't want to be mean and stop talking to you completely, but they have lost interest as far as having a fling with you and have moved on to the next. As blunt as that seems, it's true. I've seen it more than once and the situation is all the same. It doesn't take much for a guy to move on from fling to fling, but girls tend to attach themselves more to the situation. My suggestion, move on like he did. Pretend you don't have the least bit of interest and find someone else like he is probably trying to do. If he doesn't want to talk to you, why should you want to talk to him? You should never allow yourself to fall into chasing a guy, because that's when they feel they are in control and that is when the interest is completely lost.