How to reply when he says the date was nice but he's not ready for anything serious

This isn't a puzzle to solve or a signal that he needs time. He's telling you his position right now. People who want something serious with you don't usually lead with 'but I'm not ready for serious.' That phrasing exists specifically to manage expectations. He may change later, but you can't make that happen by being cool about it or waiting it out.

What he's actually saying: I enjoyed this, and I want to keep you as an option, but I'm not committing to wanting a relationship with you. That's different from 'I'm working through some stuff and might be ready in three months.' Those sound similar but mean very different things. Listen to the exact words he used. If he said 'not ready for anything serious,' he's holding the door open to casual. If he said something warmer like 'I want to take things slow,' that's slightly different and worth examining. But 'not ready' is usually final as stated.

A good reply doesn't ask him to clarify or convince you he'll change. It tells him what you're actually looking for, then gives him the choice to meet you there or not. Try this: 'I'm glad you had a good time. I'm looking for something that could develop into a relationship. I know that's not where you are, so I understand if that doesn't work for you.' You say the thing. You don't apologize for it. You don't ask permission.

This accomplishes two things at once. First, it removes any doubt about what you want, so he can't later say he didn't know you were serious. Second, it forces clarity from him. He either says 'yeah, let's see where this goes' or he doesn't. If he says 'let's just keep hanging out and see,' you'll know that means casual, and you can decide if that's acceptable. Most replies that fail here either state nothing at all (leaving him thinking casual is fine) or state a need but then soften it with 'but we can try whatever works for you' (which contradicts the boundary you just set).

Reply quickly enough that it looks like a response, not a decision you've been overthinking. Waiting three days sends the signal that you're hurt or unsure. Send it the next morning or evening while the date is still recent in his head.

After you send it, wait. Don't follow up. Don't send another message if he goes quiet. This is where many people fail. They get nervous about the boundary they just set and try to soften it with a second message. Resist this. If he's interested in exploring something real with you, he will respond. If he responds with 'let's just keep hanging out,' you already know what that means and you need to decide if you accept it. If he goes silent, that's his answer too. A week of silence after a boundary means he's not stepping up. At that point, you can move on or send one final 'hey, I think we want different things, so I'm going to stop reaching out' message. But that's optional. Silence is a choice he's made, and you don't need to force him to acknowledge it.