Sounds like me. And now that I’m almost 60, it’s getting worse.
I’d suggest focusing on the expensive items and have a place where they ALWAYS go. It will eventually become rote. The laptop ALWAYS goes in the backpack. The backpack is ALWAYS on his back or in his lap while on the bus. Not on the floor or beside him. The phone is ALWAYS in his pocket or on the night stand charging. Not in the backpack, except in the small pocket when he’s swimming. I have places where I always put stuff. To this day I always have my backpack in contact with me when traveling. For the important stuff there’s a place for everything. My wife doesn’t understand why I want a phone that’s small enough to easily fit in my front pocket but I don’t even lose it as often as she loses her phone. Sun glasses and reading glasses are another story. That’s why I don’t spend much money on them and have multiple pairs.
As for those inexpensive items, when he gets upset after losing them tell him it’s okay. I have to remind myself that it’s part of who I am. i lose stuff. When I get upset I intentionally stop and talk myself down from the anger. I give myself a mental hug. I really wish that when I was your son’s age someone would have told me that.