The best way to get your children to eat more vegetables is to put them on the table.
I am the parent. I decide what we eat. I do not make special meals for my children. They eat what I fix or go hungry. (They have never gone hungry.) Setting up the precedent that you are in charge and they are to do what you tell them is the biggest hurdle. If my children do not finish their dinner in a reasonable amount of time or they choose not to eat something, they get it for breakfast.
So, start now. You are the mom, he is the kid. You run the household, he needs to learn to live there. Make sure he sees you eat the vegetables. Tell him he eats those before he gets dessert, seconds, etc. My parents used to tell us that certain vegetables were only for the adults, because of this I still love Brussel sprouts to this day.
Don't give in. Children will not allow themselves to starve and you will be grateful for this when he has a bigger will and a bigger body.