He still wants to date other people after three months

By three months, someone either wants to commit or they don't. There's no gray area here, even if he's being vague about it. Saying he 'still wants to date other people' is the clarity. He's not confused or working through feelings. He's telling you directly that exclusivity isn't what he's after right now, and you need to take that at face value.

What you do with that information is up to you. But pretending he might change his mind if you wait longer or prove yourself isn't realistic. People don't usually shift from 'I want options' to 'I want you exclusively' just because more time passes. He either decides you're worth committing to, or he doesn't. Three months is enough time to have made that call.

The move here is to tell him what you need in a relationship, not ask him to explain himself or promise he'll reconsider. A good reply does two things. First, it confirms you heard him and understand what he's saying. Second, it tells him you need something different and you're not waiting around to see if he changes.

Don't frame it as an argument or a punishment. Frame it as a simple mismatch. You want exclusivity. He doesn't, at least not with you right now. Those two things don't work together, so you need to know what he wants to do about that. The reply should be short enough that it doesn't feel like you're pleading or over-explaining.

Don't sit with this conversation for weeks. Once you know where he stands, respond within a day or two while it's still fresh. The sooner you say something, the sooner you both know what's actually happening.

After you send it, give him real time to respond. Don't follow up the same day or send 'just checking in' texts. If he doesn't reply within a few days, that's also information. Either he's avoiding the conversation because your boundary made things uncomfortable, or he's not interested in having it. Both of those mean you have your answer. At that point, you stop waiting and move on.