Talking to his doctor may be what you need to do. However, could it be that he is constipated? I know my son does not eat when he is constipated on occasion. If he is constipated maybe trying some foods that help with digestion. More vegie related or fruit related items. Get him foods he likes to eat and allow him to pick his foods out at the store. Making him part of the process. He might be more apt to eat especially at daycare.
Another thing... I know my son's doctor told me when my son was younger that he will eat when he is hungry and not to sweat it. I noticed the older he gets the more truth to this there is. That was hard to hear then, but you know I think the man was pretty smart. My son has never been under weight or over weight except at birth. LOL! He was 10 llbs. He's pretty healthy.
One last thing I would make sure he has lots of liquid if he isn't eating. Maybe protein shakes and of course lots of water.
Good luck...to rest your mind though I would call the doctor and talk to the nurse or the doctor about your concerns.