My son (5) is literally starving himself

He has completely given up on food. He has an appointment tomorrow. Since over a week ago, he came home without eating his lunchbox. Then he would only eat treats..then snack..then only yogurt..now nothing. I don’t limit his food intake. I’m not lying when I say he literally doesn’t eat anything. It’s past 2 pm, he’s only eaten one packet of fruit gummies. He’s drinking water and not dehydrated, urinating, has tears.

I’m now super worried because he didn’t even have the strength to walk more than 15 minutes for trick or treating. Yesterday he slept 14 hours. We skipped school. He played for a couple of hours sitting down. Then said, he’s sleepy. Went to his bed. He was just lying down on bed. So clearly, he’s lost all his strength. However, he’s screaming loudly and looks fine otherwise. He does all his pranks, silliness, voices when he’s in bed. No fever or anything.

I don’t know if it’s behavioral or health related. We will of course rule that out but this morning I noticed his underwear was so loose. He’s pretty skinny to begin with. Before this, his energy level was okay, not like super energetic.

Any feedback?