But why do you think it’s him? Ton of people have exes they are dwelling on regardless how they were treated.

But what if it’s him? He made a new user name, which people do all the time and is posting about his other ex.

By replying to him (thinking it’s him) you send a message that you care that he is thinking about ruined relationship with his ex. You think he is posting for your sakes but he might think you are reading and replying for his sakes.

If you think it’s him, replying asking why is he not over his ex might appear just as “immature” as him posting whatever he is posting. It also might appear as you are jealous of his ex and still isn’t over him. Things could appear different ways to different people

I hope time will heal all this. It just seems to not contributing anything of value to your life