Kinder ONLY eats McDonalds?

My son will only eat chicken nuggets from a restaurant such as McDonald’s. I’ve been getting him McDonald’s nuggets because that’s like the only food he eats. Sometimes he claims he’s soo hungry but when I get him food, he only eats a few bites and then says he’s full. My son has barely touched the 1% tile in weight for those who are around his age. I can pretty much count on him eating most if not all the food if I get him McDonalds. I don’t want to always be buying him McDonald’s because it’s obviously not the healthiest, but I do want him to eat. He’ll absolutely refuse food if it’s not nuggets from a restaurant. He cannot afford to not eat because he’s already very small for his age. What do I do?? I got him tested several times and have had him be seen by a pediatrician and specialist but no one seems to have answers. I’m in the middle of switching pediatricians, but has anyone else have experienced similar issues? If so, what was the outcome??

Even food that we already have at home that he says it sounds good, he’ll only eat a few bites and says he’s hungry. He’s so small that everyone doesn’t believe me when I tell them he’s 5