I think Kav would be your man - he is the resident kettlebell guru
Not sure if I'd go that far, but thanks.
It entirely depends on the strength of the boy you're buying for - a 16kg bell might be just right or if he's built like a tank he might scoff at it. The thing to bear in mind with kbs is it's the combination of cardio and resistance that gives you a good workout - you don't necessarily need to be lifting a massive weight to get good results. So consider his level of fitness and how strong he is, and take it from there.
To be honest I don't know what's a "correct" weight for a still-growing boy, but for reference I currently use anything from a 16 up to 32kg bell depending on the exercises. If I was to pick a happy medium that would do me for most exercises I would probably go for a 20 or 24kg bell.
It's easy to get a great workout even with fairly light bells by repping to failure, so there's no need to go OTT by getting too heavy a bell. Like all lifting, proper form is more important than heaving a heavy weight and straining something. The strength will come with the repetition if he sticks with it, and then he can move on to a heavier bell if he's into it. Gauge it yourself based on what you know of him.