Explaining to your kid why friendships end

My son has been heart broken the last several weeks because his best friend since preK doesn't want to be friends anymore and he doesn't understand why or what he did. He noticed the change the beginning of the school year. They're both 9th graders and he came home saying his best friend acts weird whenever he sits with at lunch. His best friend started hanging out with other people and he asked if he did something wrong and his best friend said no he just wants to be friends with other people and out of nowhere blocked him. He was hurt about it and I did try calling his best friend's mom to figure out of their was a falling out or something she knows the lead up to this but she said her son is just going through a phase right now. I've tried to explain to him that sometimes friends out grow each other but that doesn't seem to have helped at all. I know friendship break ups hurt a lot. I feel like I don't have the right answers to why friendships end.