That's a toughie because on one hand you have to be supportive of your growing son's chance to make his own decisions in life, even mistakes. On the other hand he is only sixteen and he wouldn't be ahead academically without the support of his family. It's very important that he not jeopardize his future with bad decisions this close to graduating. At sixteen I would be open to being emotionally supportive in case the relationship turned sour, but I wouldn't be very worried about it. Who marries their first sweetheart? It's rare. And heartache is a right of passage. I would worry at this point about an unhealthy or high-pressure relationship where they're moving too fast twords "engagement" or a family or an unhealthy relationship where his time becomes exclusively monopolized. Of course they will spend a lot of time togeather. That's not what I mean. You know what I mean.
