Most formula is make to give to infants. Make an appointment with a Nutritionist, maybe at a local hospital? and talk it over with her. If your Pediatrician says do then you can do it and know it is the correct thing to do too. the formula for older children is chemically different than infant formula, it has more calories for one thing. You might look at the nutritional suppliments like Ensure too. The kids like the vanilla and strawberry flavors but the chocolate is nasty....
Food is also a touchy subject too. He may just not like the foods being offered. My friends little boy wouldn't eat anything until she gave him Mango's, her daughter took off eating Avacado's. K only ate those little puffs that are flavored like tomato and spices. J, on the other hand ate anything you put in front of him and he still does at 3.
Get some cookbooks from the library and try them out. They have lots of ideas I would have never thought of and they quickly became by fav's and then I had to go and buy them at Hastings.
Baby and Toddler Meals for Dummies
Idiots Guide to Feeding Your Baby and Toddler