Please give me advice 🙏

Hello, I just want to ask because I'm stressed with my 3-year-old son, he doesn't eat rice, but when he was 1 year old, he eats it, now he's 3, suddenly he doesn't want it anymore, then suddenly only milk supports him. He doesn't want milk anymore, I don't know what to do, he's losing weight, milk just supports him so that he's always full, suddenly he doesn't want milk, what can I do? but he eats fruits but I prefer it because he learns to eat rice. please give me advice since he's my only child I have no idea I'm a first time mom.