Low carb snacks could be
Almonds (chopped and roasted, add a bit of stevia sweetener if you want makes them crunchy and brings out the flavour)
Other nuts.
Bite sized cheeses (baby bells)
Cucumber sticks, green bell peppers with a mayo - if he gets along with carrots that can work too, but they're higher in sugars.
Full fat yoghurt with a bit of sweeterner (greek yoghurt normally prefered by most on here)
Small amounts of berries as they tend to have less effect on most peoples BG than do other fruits.
Sugarfree jelly with whipped cream is a favourite among some here too.
Small salamis or cocktail sausages.
Is the kid on insulin or is he a kid with type 2? Guessing insulin, yeah?
If so, he can have some carbs for snacks as long as his levels are not high I think. I am an adult of course, but I think I can get away with 5-10g without a spike if I am not high to begin with. Most sensible portions of above will stay within that limit. So would a quarter of an apple, half a thin slice of bread...