What do you do when your child doesn't want to eat no matter what type of food you serve for that particular meal?

My Elderly son picky eater, sometimes, he eat one bite or 2bite in each meal, then he say to me i want milk, so i just give milk , 1day almost 10bottle he drink because he so hungry but he dont want eat , when checkup time i tell to nurse and then nurse give me a suggestion to give this kid a vitamin which is lysine, then my sons keep eating the vitamins until 4-5month , then today no more vitamins, because he eat more than my expectation, im happy because my sons eat 3time /day, so no need vitamins anymore , but he still picky eater, because he will choose whats he want or not,