How to mention he dominates every conversation with his stories

One long story from him is normal. What matters is the shape across several exchanges. Count how often your message gets a direct response versus a pivot into his own experience. If your questions get answered with a lead-in to something he wants to tell you, that's the signal. Also look at what he does with your short replies. Some people who overshare are just filling silence because they're anxious, and they'll ease off once they feel safe. Others barely notice your input at all. You can usually tell which one you're dealing with by how he responds when you ask him a direct question about himself and then wait.

Pay attention to topics too. If you mention something you care about and it gets absorbed into one of his stories within a line or two, he's probably treating the chat as a place to show off rather than connect. That's worth noticing before you decide how much to invest.

The reply needs to tell him exactly what you've noticed, in terms of what happens in the chat, not who he is. Something that refers to the back and forth, like how your last few messages never quite got their turn, lets him respond to the pattern without feeling attacked. Keep it light enough that he can laugh or ask about it. Then move straight into a question about something specific he hasn't said yet, so the next move is his. Don't add a second complaint, a summary of past grievances, or a request for him to change. The point is to open a door, not to win an argument about the last three days.

If he responds by apologising and then immediately telling a new story, you've learned something without needing a second conversation about it.

After you raise it, leave a real gap before judging. Two or three days is enough to see whether his next messages change. Watch whether he asks you questions that go beyond the surface and whether he follows up on something you said instead of jumping to his own next topic. If he improves even a little, keep the conversation going and let it get more balanced without bringing it up again. If he slides back within a message or two, you can mention it once more, briefly, and see whether it lands. If the pattern survives both attempts, you have your answer. You don't owe him a third chance to become a different conversational partner, and you're allowed to stop investing in someone who only plays one side of the exchange.