Put him in the chair when it's not his mealtime. Praise him for anything he does that you want. ("You did a good job of putting your legs in the holes!") Pay attention to him as long as he stays calm. Read him one of his favorite books. When he kicks up a fuss, simply look away from him and stop reading. When he calms down-this may take a few minutes-look at him again, smile, praise him for being calm, and resume reading the book.
Do this two or three times a day for a few days. At that point he should be calm and even happy when you put him in his highchair. Then you can start feeding him while he's seated. But keep in mind that you can't expect any toddler to sit still for more than a few minutes.