If you already know he’s sneaking out, a window alarm might not do much. Next time, wait until he’s out, lock the window, and have a chat when he comes back.
Consider setting up window sensors that notify you when they’re opened, and maybe add a tracker to something he carries regularly. Just be aware that if he finds out, it could damage your relationship with him. Some kids just sneak out to get a bit of freedom, and it might not mean much more than that.
Anything that can detect when a window is opened should work. Detex makes commercial options, but you probably don’t need that. Most standard alarm systems will chime or alert you when a sensor is triggered.
You might try window locks. I have some installed to prevent my kids, who sleep on the top floor, from accidentally opening the windows. They’re easy to use but still secure. Here’s a link to the kind I use.
I use a Ring alarm, and it offers customizable alerts and chimes for each device. For example, my patio door chimes when it’s opened, and I also get notifications on my phone if any basement doors are accessed.
For those speculating on why he’s sneaking out, making comments about ‘freedom,’ or even messaging me with threats to involve authorities, kindly back off. To clarify, he’s been sneaking out to do drugs and vandalize property with some so-called friends.
He’ll probably find a way around any window alarms. Consider putting one in the hallway outside his room too, in case he tries to sneak out through the front door next.
Back in the day, we were practically kicked out of the house after dinner to play until dark. Tracking down where kids were was a whole different issue, but it was more about letting them roam freely.