It was pretty clear to me, his feelings of friendship started to drift after Ryo got a girlfriend and they started to get closer, then more intimate.
Yudai started to get involved with everything about kakeru and the kiss in the art room was literally his love exploding at his face, remember, he was pressuring kakeru about the guy he likes and just when he thought he was about to blurt it out, hekissed him before he could.
The story was perfect IMO. It was subtle yet very clear, bittersweet and not vulgarly obvious.
Yeah, that was a crucial moment, but I'm the kinda person that still likes moments with less ambiguity and to get more of the other's perspective. Regardless of the transition of feelings from one to another, I still feel like a more of them in present time would have been a actually perfect ending.
Indeed we only read the story from kakeru's point of view but that's what has built the atmosphere in the story, it would have felt totally different from yudai's Pov.
As for their story starting from their reunion, I'd rather have a sequel than a prolongation... But that's my opinion, I really loved this story.
It was so good, but we really needed more of their present time :( and maybe more of Yudai's perspective. Like, when did his feelings change? The transition between being in love with Ryou to falling for Kakeru. I really need to see that in this kinda stories cause otherwise is not that convincing and it feels rushed:(