Would exercise be counterproductive for an underweight person if he did not meet the daily nutritional intake? what if he did meet the requirements?

A workout might: Cause an increace in appetite.Focus on protein, fruit, healthy fats...Shun carbs. Good luck, killer.

It is ALWAYS good: Jason, I am a physician who supervises the health of athletes. A fair number of elite track-and-field athletes are "underweight" by the books. There is no reason whatever to strive to meet some beancounter's definition of "healthy". If you're exercising, your body will give you a better sense of what you really need to be eating. You can't gain mass while you're starving; eat sensibly.