By all means, start cooking lessons with the things your child likes to eat (which probably means breakfast and sweets). But somewhere between that first batch of brownies you make together and the high school graduation cake, you should make sure he knows how to cook dinner.
When you choose recipes to make with your youngster, think about skills he will be learning and emphasize the fundamentals. Everyone needs to learn to make a white sauce, roast vegetables, steam vegetables, prepare salad, cook pasta, mash potatoes, make salad dressing, bread poultry, cook rice, chop and saute onion, make stock, brown meat, de-glaze a pan, build a gravy, etc. Consider having a list of recipes that will teach these skills, and working through a few every month.