If you are on the MYW dining plan, and your son is 9 or under, he will be relegated to the kids menu. If he is over 10, you will pay $35 per day (same as the price for an adult), and he could order off either menu, although you might not get your moneys worth if you plunk down $35 day per day for him and he ends up exclusively eating from kids menus.
If you are not doing MYW, and are paying as you go, your DS can order off either the adult menu or the childrens menu, and will pay the price listed on the menu for the items ordered.
If you are paying as you go, sometimes the buffets are a good deal when you have an adventurous eater, and they qualify for the kids price. Then they can try whatever they want, but you are paying the lower price. Boma might be a good option for your family to consider if you are in this situation.