Should worry about my 16yo brother?

Hi everybody, My name is CC. I'm 20 and I live at college about half an hour away from my family. I'm worried about my 16yo brother. He's grown very tall and skinny in a very short amount of time. He was a chubby child, and my mom and his soccer coaches used to bother him about it and encourage him to work out more often and eat more healthy. He counted calories for a bit, lost weight, gained muscle and became a very accomplished athlete. They were kind of mean about it though, and I worry the criticism may have negatively affected him and made him obsessive. He doesn't often join us for meals, although I'm not home super often. He eats sort of secretly in the middle of the night, and if I ask my parents if he eats enough, they say he just eats in the middle of the night. This again, might just be what a teenage boy does. However he also is very sensitive about eating food. He will snap at me if I ask, even jokingly, if he eats enough. Over the last year, he has grown very thin and he exercises very often. He plays tennis very seriously and goes to the gym several times a week. This definitely could just be that he's growing very fast and wants to be fit. I know our mom feeds him a big breakfast (I haven't personally been up early enough to see if he eats it. I assume he does or my mom would be worried.) He completely skips lunch at school but usually (according to my mom) eats a big snack when he gets back. A few weeks ago, he actually passed out when he didn't eat an after school snack and fell down the stairs, which scared my dad. My brother, like my mom, passed out a lot as a kid from talking about blood or getting a shot, and I believe his blood pressure is low. Everybody has a different story. My brother claims he didn't pass out from hunger, but instead tripped and hit his head and passed out. My dad claims he had a seizure (which nobody has any history of, and my dad hasn't seen my brother pass out often enough to know what it looks like). My mom believes it was from hunger because he didn't eat all day and plays sports after school. Everybody was freaked out, but it hasn't happened again, and it just got kind of dropped. He doesn't seem to avoid certain kinds of food too much. He certainly doesn't like sweets as much as he used to, and he won't usually indulge in dessert. He obsesses a little over not eating too much junk although he definitely could afford to. Again, this could just be a health-conscious teen. On their own, none of these are entirely concerning (except maybe passing out, but he has always passed out very easily, and the reason is debated). However, this morning I found a bottle under the sink of knock-off weight loss pills that look too sketchy for my parents to have purchased them. When I asked my mom, she said that nobody had claimed them. Since my dad has no reason to hide them, they belong to my brother. I don't have any experience with eating disorders, and the evidence doesn't seem solid enough to bring up to my mom. On top of that, my parents are weird and uncomfortable about mental health problems (I hid my own seasonal depression and anxiety from them through high school and only found a therapist in college). If I bring it up to my brother incorrectly, he will become defensive. He and I have a pretty good relationship, but he sometimes shuts down about certain topics if he isn't in a great mood. I don't think he would confide in me if he were having self image problems. I will be home for winter break for about 5 weeks where I will see him every day, so I will be able to see whats going on. Should I be concerned or am I worrying too much? What should I do? Thanks for your help, CC