@elefantine97 have you read Night on the Galactic Railroad? this story is largely inspired by and references it, from the questions of what happiness is and what life is worth living for down to the boy being the equivalent of Campanella, a symbol of kindness without agenda, and a character whose final destination is not the same as the main character's, but manages to help him with these questions in the brief time that they ride the train together. It's a really well-done parallel.
In terms of what the teenager meant to our MC, I think not only is it about the boy's youth, but also this unconditional kindness that he gives to the MC from the very moment they met. Kanou is so wrapped up in the conflict between his sexuality and society that having someone be so accepting without needing to know all the details was probably a real weight off his chest. Basically, everyone needs a friend, and because of his sexuality, Kanou always kept people at a distance, even though he constantly craved contact anyways. He was able to make a connection with the kid (even though he still felt like it was temporary) and probably mistook that for a "crush".
I'm glad by the end, Kanou was willing to maintain the connection (inviting the kid to the store) instead of just pessimistically thinking that it was only circumstantial and fleeting. And I'm glad he was able to break down his walls and let himself feel love again.