I have a seven year old that is 44 inches tall and weighs 43lbs! I know what having a thin child is like and she too will come home with half a lunch box full still of stuff. Yes not being overweight is awesome for children with the diabetes going on and the health issues. I know my daughter is taking in the calories she needs though at some point during the course of a day.
My daughter is one that if she is hungry, SHE CAN EAT! Typically on the walk home she finishes her lunch, then wants a snack when she gets home and is very hungry at dinnertime. We have read books about why your body needs healthy food and what you need to have energy to do the things she likes. She really got into learning about the body and how things work. She will even limit herself if she had a junky snack and say "that is all I need today for junk". So educating instead of nagging can help. Have his Dr sit him down and talk to him. If he is into a sport, have the coach talk to him about how important eating and staying fueled up can be. I can say if you see it is for attention, try carving out special time for him each day, say nothing about the food. Even boys can get eating disorders so pay close attention what the reasoning is,
One rule I have, you don't have to finish your meal, but do not ask me for anything later. If they ask later, dinner is in the fridge and will get reheated.
Try taking him to the store and allowing him to choose his snacks and lunches.
We just started doing wraps, at my daughter request and she loves them. Yesterday I got a tortilla, put honey mustard and turkey and cheese and she ate her lunch for a change!!!! She had favorites then gets tired of them, so I let her try new favorites. Same with my four year old son. I think they get tired of what they liked and need things changed.
You cannot force a child to eat, as long as on the side he isn't getting desserts or junky stuff without eating healthy first then I would make sure he is getting a daily vitamin, maybe try protein smoothies, milkshakes with extra vitamins and getting the nutrition he needs. My kids loves smoothies for breakfast and with enough fruit and yogurt and all that they are very healthy way to start the day off. If his Dr isn't overly worried and he isn't lethargic or grumpy due to not eating then I wouldn't worry.
If he truly isn't hungry then what can you do?
If he is just picky then that is another thing.
Have you thought of going to school one day and having lunch with him? I know that was an eye opener for me, kids are socializing, in a hurry to get out to recess which is why I think my daughter isn't eating so much, she is too busy worrying about getting to recess with her friends. I go eat lunch with her at school maybe once a month. It helps you gauge what is going on, why he may not be eating and how things are run in the cafeteria. Would he prefer to get hot lunch on occassion at school?
I also let my daughter pick what she is going to pack for lunch. That way it was her choices and she is more apt to eat it. Good luck, I hope things change for him and you.