Just wanted you to know you're not alone. Our seven-year-old also hates to eat. He was a super eater until he was two and then everything hit a brick wall. He's extremely, extremely picky, with maybe five meal items he'll consider eating. Zero fruits or vegetables and will simply not eat if presented with them -- in fact, will storm off to his room if we insult him with them. We can only get those in through apple juice that has veggie juice in it. He won't drink smoothies. Doesn't have a sweet tooth. We joke that the best Christmas gift we could give him would be to tell him he never had to eat again. He'd be thrilled!
Our son's doctors are also concerned about him dropping percentages (he was 99% when born, so he had a long way to fall), so I understand the pressure you're under so well. We've been told by two different doctors to give him as much fat as possible since he doesn't eat much, because it's critical to brain and body development at this age. Not an easy task when I'm maintaining on Weight Watchers! It's so counterintuitive. The doctors are far less concerned about nutrition and just getting our son to eat.
We've tried rewards, punishments, making custom meals, not making custom meals. Tried pressuring him, tried being very low-key and "whatever." I have yet to find the key to getting him to eat.
I also know what you mean when others say they'll eat when they're hungry and you have a child who truly won't do that.
I really wish I could say what works in our situation, but have had no luck. Our son already sees a psychiatrist and behavioral therapist for ADHD (and no, his medication isn't the cause of the eating problems ... started well before he was even on them and are at their worst when the medication is out of his system). We've asked about eating disorders and they say no, just reiterating that we give him fatty foods. Haven't decided whether it's a good or bad thing there's no definition for what's going on.
Good luck! I hope your situation is just a phase. At the very least, I've got to think our boys will eat when they reach the teenage years.