A while ago someone on the forum posted a msg about cheese being able to prevent hypos and trying this out on myself before bed, it appears to make my bg levels rise a bit so this be useful for your son. If he likes a cracker with some dairylee triangle cheese on it nice and thick, topped with a heaped teaspoon of peanut butter that should be a good hypo preventer that will make insulin struggle to lower bg. Worth trying just to see.
The other way is to get cgm and set up alarms. Another way is to download the app Sentector which listens to sound and can be used between 2 mobile phones.......One for the user and one for the listener.