A long time ago I happened to come across this shounen ai/yaoi manga about this young adult couple who lives together. One day one of them suddenly turns into their child body. Nothinh else changes, just the appearance of now having a child body. I don't remember how this happened. Also, a brother of the guy who didn't turn into a child came to visit them. And that's all! I think all of that was only the first chapter and I've always wondered what will happen. I don't really read yaoi but the plot sounds interesting.
Please help me find the title!!!!!! A million thanks if u can!!! <3

It's probably Sayonara, Otoko no Ko.

It is, tysm!