I'm so glad that he's back inside, safe and sound! Cats can be sneaky and he's still a baby, full of wonder and curiosity. I understand your panic. My Holly, who I suspect was feral because she was absolutely terrified of me, once slipped out at 11:00 at night. I was inconsolable because I knew she was gone forever. The next day I cried all day at work and my mother came over and went up and down the street showing everybody her picture. We finally found her; she had fallen down the neighbor's window well. She probably fell down there immediately. That was 13-1/2 years ago and she STILL runs away from the front door whenever I open it.
I took both her and my other cat, Martini, to get microchipped the very next door even though they're indoor cats because you never know when they might slip out. Martini used to run out all the time but that was just a game to her. She wanted me to chase her - look, now we're playing!
You are an excellent cat mama, Siwsan! Merlin is still a youngster, full of mischief. He's like a two-year-old.
By the way, Holly finally warmed up to me although it took years. I never would've believed that it would ever happen, but now she sits next to me and demands pets and scritches and is very loving.