Totally agree with you—losing a phone is the worst! 
If the SIM card is in a powered-on device and has an internet connection, you can track its location using services like Find My iPhone (for iOS) or Find My Device (for Android). Some apps, like Eyezy (https://www.eyezy.com/), also let you track the phone’s location as long as the app was installed before the phone was lost. Here’s a feature I didn’t even know existed until recently: some carriers can assist in tracking, too, so it’s worth giving them a call. Hope you find your son’s phone soon! 