I don’t think it’s that immature to not respond. It sends the desired message and things are still at a very low-impact state. You never asked him to text you. You never invited him. You gave out your number reflexively, and he knew it. That’s why he sent the text. By responding, you were saying you were open to a next move. Now, if you want to say no, it will take a little more effort, although you can still get away with just ignoring future texts.