Perhaps he's having some personal issues or problems that are taking up his attention and preventing him from watching your story right now.
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.
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.
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.
Maybe he still has some interest in you or just happens to come across them.
Maybe he's just curious about your life or finds it interesting.
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.
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.
You don't need to feel too bad. It's his choice and it doesn't necessarily reflect on you or your story.
It could be that he finds your updates engaging or maybe he has a crush on you and wants to stay connected through your stories. Or perhaps he just has a habit of following people's stories without a specific reason.