Oh my heavens. You didn't do anything wrong! Clearly he liked you, to the point that he arranged to come to your home town to see you again.
One possible reason for his actions is that he's scared of becoming too close to another person. I call this intimacy phobia. The fact that he liked you instantly could have terrified him. For an intimacy phobic, realizing that he liked you enough to come to your town the next day would scare the daylights out of him. He may have run for cover in order to escape his feelings and dilute his budding attachment to you.
Let's say he's not an intimacy phobic. What else could explain his behavior? It's possible that he acted coolly as a way of protecting himself from the pain of leaving you. You live in the UK and he lives in Germany. Perhaps he was cutting off in order to avoid becoming involved with a woman who lives so far away.
At this point, you have three choices. If he's an intimacy phobic, your contacting him will intensify his fear and he'll run even farther. In this sense, contacting him would be diagnostic. But, be prepared. If he has the problem I think he does, he could come close and run away again. You'll know that he has this problem if you reconnect with him and soon start to feel like a human yo-yo.
What if he's not an intimacy phobic and he just pulled away because he didn't want to connect to someone who lives in another country. You could contact him and say that you noticed he went from friendly to cold. You could tell him that you felt a mutual connection and you wonder if he pulled away because he doesn't want to become involved with someone who lives too far. Tell him that you don't want distance to stop you from exploring whether you're right for each other.
What do you have to lose? If he's an intimacy phobic, this will push him farther away, but, if he is, he'd be lost to you anyway. If he isn't an intimacy phobic, then you just might be able to strike up a dialogue and connect despite the distance. If you are right for each other, the distance won't matter because eventually one of you will move!
Your third option is to wait and see if he reaches out to you. If he does, then you will discover whether he comes close and runs away again (meaning he's an intimacy phobic) or whether he was cool because he thought a long distance relationship wouldn't work.
Choose the option that feels right for your personality.