I tought my little sister to cook (shes 13 and the same kind of situation)
Home made pizza is good, he will learn to make the dough and pizza is yummy

muffins are easy, but they require measuring and stuff, which he could use later in life to make other things. Get hime to prepare a family meal. Let him do it all on his own. Give him a list of foods to choose from, and leave him to it
foods that are good for this are;
pizza (home crust)
Steak (or chicken) with potatoes and side veggies
a roast
pancakes(breakfast)
Cakes
Muffins
ribs (again with sides)
stuffed peppers
Shepherds pie
tons of others, i hope this helps
good for you to want to teach him these things, cooking is a lost art