I think that's normal when people lose weight in a hurry, they do look dreadful at first. DBH lost weight in a similar fashion and now only eats around 2000 calories a day, as he is a keen athlete. He is 5'11". I preferred him when he was cuddlier, he is only half the man I married.
In fact he is probably a good two stone lighter than he was when the photo on my profile page was taken. However, he has loads of energy and feels fitter. I notice he feels the cold more though. I would suggest that your son needs to be on at least 2500. I can see why you are so worried though, I was. It took us a while to get used to sorting him out a new eating routine, his change of menu affected the whole family and still does, he is very set in his eating habits now. He has also virtually given up alcohol, which hasn't done much for our local social life!