Help! My nephew is such a picky eater. :( How can I make sure he gets all the nutrients he needs when he only wants to eat Jollibee!

If he's a really picky eater, I would start by mimicking the menu of Jolibee but make sure that it is homemade. It wouldn't be the healthiest but at least you'd know what's in it and it'll be a good transition for him to other dishes.