16yr old son has lost 40 lbs in less than 6 months. He is 119 and 5"9. He is very cautious about how much he eats. His pediatrian says it is a phase.?

Continued. If his answer is "yes" and he really does think he is too fat, then that is evidence of an extremely distorted body image, and i would advise seeking help right now, before this goes any further. If his answer is "no" and he plans to maintain his weight where it is, then you can wait and see, so long as he doesn't get any thinner.
Maybe. If he's lost 40lb in 6months, he's lost approximately one and two thirds pounds per week, which is a good weight loss rate. His bmi at his current height and weight is around 17-18, which is very thin. If he's just trying to be healthy, that's fine. If he continues to lose weight he will not remain healthy, and he's at risk for an eating disorder. Have you asked him if he thinks he's too fat?