Why is my kid not eating anymore?

He used to eat so much. Now for the past couple weeks he just throws everything on the floor. He’s always whining now too, which he rarely did before. I’m guessing because he’s hungry but he just won’t eat.

The only time he eats is in the morning. After that, next to nothing. I made him what used to be his favourite last night for dinner(Chicken, mashed potatoes & corn) he just chucked it all on the floor.

I’m at my wits..I don’t know what to do.

Anyone else have this problem?