How to make my 2 yr old son to have more appetide to eat?

Picky eater. Don't let him fill up on snacks & junk food. Encourage him to try a variety of foods. Be a positive role model: parents who refuse to eat certain foods can't expect their children not to refuse them too.
Don't force it. If his height and weight are within range, he should be okay. Check with your pediatrician; they are used to answering concerns like this. Bring specific information like portion size and types of food, since they will evaluate his nutrition at the same time. Remember he's only two, and won't have an appetite like an adult. Is he otherwise well?