It could also be that something in the story made him uncomfortable. For example, if the story contained topics that he didn't like or disagreed with, he would choose to stop watching it. Also, perhaps he accidentally clicked away and then just forgot to come back to it. There are many possible reasons for this situation.
It could be that the content of your story doesn't interest him anymore. Maybe the topic or style changed and it no longer appeals to him.
Maybe he's just too busy right now and doesn't have the time.
He could be busy. His schedule might have become packed all of a sudden, leaving him no time to check Instagram stories, not just yours but perhaps any at all. For example, he might have started a new job with long hours or have a lot of personal projects to handle.
Perhaps there was something in one of your stories that made him/her uncomfortable. It could be a topic, an opinion or even just the way you presented it. So he/she decided to stop watching.
It's possible that he lost interest in the content of your stories. If your stories have been about the same thing over and over, he might have gotten bored. Another reason could be that he has found other sources of entertainment or information that he prefers.
It's possible that he lost interest. Maybe your stories have become a bit monotonous or not as engaging as before. For example, if you always post the same type of content, he might get bored.
Maybe he still has some interest in you or just happens to come across them.
Maybe he's just not interested in your content anymore. People's interests change over time.
Maybe he got bored. If your stories have been similar in style or content for a while, he might have lost interest.
There could be many reasons. Maybe he's just busy lately and doesn't have much time for social media. Or perhaps he's trying to distance himself for some personal reasons. It could also be that he has found other more interesting content or people to follow.