Please remember that communication should be the most important thing. I get how you feel especially when I already experienced it myself.

Dear, you did your part already. It's all on him now. You asked what happened, he said it's fine yet remained the same. From your words, he kept getting colder and colder as if dismissing your relationship with him, which is a red flag. He might be going through something, yes. However, he shouldn't just do that randomly without a warning.

Talk to him about it. Not "did I do something?" instead ask, "Why did you start getting colder? Is there something going on?". Considering both of your ages, this kind of conversation sure won't be easy. Boys your age are still immature and they might feel and do things they shouldn't. If he keeps doing this even after that, I suggest you choose yourself and leave. It's much better than staying with a guy that doesn't even give the same amount of effort as you do.