You've gotten some great answers here, S.. I don't really have much to add advice-wise, but I do have a couple of anecdotes -- my son is 16 and has never been "into" food. I've been waiting for that "eats everything in the house" stage to kick in, and no luck so far! He likes food fine, he's just not a big eater. He's skinny as a rail and we have a hard time finding jeans that fit his waist AND his height (he's tall). He's a homebody, doesn't go out much, but very secure and happy. He eats when we have a sit-down meal, otherwise he just eats when he's hungry, and he's fine! Healthy, happy, all that good stuff.
On the other hand, when I was growing up my oldest brother went through a drastic weight loss when he was about 16. This was back before anyone knew much about eating disorders, or even all the ins and outs of depression (1970s). They had him tested for all kinds of things, including tapeworms. Come to find out, he was anorexic. He had been a pudgy child and early teen, and was also going through severe depression at the time, so he took to throwing up all his meals in order to lose weight and, I guess, feel more in control of his life. Thank God they discovered what was going on! It was a long, hard struggle for him, but he is 56 now and although he still has food issues, at least those are mostly being fanatical about health foods -- and he's still alive!
So -- probably this is completely normal and nothing to worry about, it's just your son's metabolism, or whatever. But it wouldn't hurt to take him to the doc and have him examined, tested and, if necessary, talked to about good nutrition. But I know from experience -- nagging about eating won't make it happen. You've just got to let him be responsible for himself!