Tofu's ready to go right out of the package, but it's fairly bland. First see if he likes it just cubed--and remember that kids need to be offered a new food about 10 times before they take to it! I like to cube it, then marinate it for a few hours in tamari (soy sauce--DON'T use La Choy or other so-called soy sauces which have sugar and "caramel coloring" added; it should just say "water, soy beans, salt"). Make sure the cubes are small enough to avoid choking.
You can also mash it with seasonings--I make a spread using dillweed, mayo, and a little veggie salt or tamari--for sandwiches or crackers.
Best of luck, and congrats on feeding your son healthfully (hopefully you eat it, too)!