Madoka. It's not your fault. It's not their fault either. People don't forget others that easily unless they are unable to even remember their own selves. And you're not that type either.

It's unfortunate that the situation is as if for you he has never been there and that for him you have forgotten him, but it's not something caused on purpose by either of you. Unless it's a prank. So you don't need to make it up to him, all you need to do is just relax and move on. Get to know him again and everything else. Without care of whose fault it is or who has the right memories.