I know you may not want to give up, but not everyone in your life is going to return the feelings. I know it's easier said than done, but you need to move forward. Instead of focusing on one boy that has stated that he is not interested in a relationship, why not start anew? As well, you said himself, he doesn't know you. Most people don't appreciate someone that they don't know being able to text them. And many people just can't be interested in someone that they know well. So for now, it's best to just let him go. Try and turn your attention to other things for now to keep you distracted, such as studying for your GCSEs. And just give yourself time.
I know you don't want to give him up, but he messaged you with a request to stop messaging him as you currently are. Maybe in the future, when he gets a chance to know you in person, things will work out. But for now, he seems uninterested. Maybe he doesn't have time to persue a relationship (especially if he has his own set of tests to take care of), or maybe he has his eyes currently set on someone else. Whatever the reason, he has requested that you stop, and I strongly suggest you respect that request. If you continue to do so, it will only put you in a bad light in his eyes.
As for not eating or drinking. I know it's hard when you're depressed, especially with so much stress going on (with your GCSEs and all). But eating a proper meal can really help you feel better. If you can't eat large meals, then try to eat several smaller meals throughout the day. You're doing your body no favors by not giving it the nutrients you need. So try and eat several small meals, if that makes it easier for you. Snack on veggies through out the day, or eat a small bowl of something and come back and eat something small an hour later. You may not feel hungry, but your body ~will~ appreciate the food.
And also, another thing that might help you is exercise. It can help you relieve the stress, and just make your mind clearer and happier with yourself. I know many people that find that exercise helps them cope :). Good luck, alright? I hope it all works out for you :). Who knows, maybe your sights have been so focused on this one boy, that you're overlooking someone that's much better for you :). Best of luck!