He's at risk of being underweight, but unless he has lost a lot of weight, lost his appetite, has an unhealthy diet (someone I used to know lived on soup and crisps) is vomiting, is tired or otherwise unwell, then a low BMI can be normal, especially if he's otherwise well and developing normally.
Remember, at 16, he's still growing. But if you're concerned (and/or if he's concerned), then visit your doctor and present this specific concern to him/her, if only to put your minds at rest.