First and foremost, it's common for birds to not accept fruit, or to be very picky with it (like only eat apples and nothing else). Don't worry about him disliking fruit- it's normal. Continue what you've been doing as far as preparing it in different ways. Perhaps try serving a different part of the fruit- for example, if you try feeding him bananas, then offer him the banana peel for a change (washed, of course), or the orange peel. Sure, the peelings of fruits don't offer much in the way of nutrients, but it can help him get acclimated into eating them.
Secondly, most of the best vitamins are found in your leafy green vegetables, stuff like spinach, broccoli, and romaine lettuce. It's great that he does eat these, and as long as he is, I wouldn't really worry about any sort of nutritional deficiency.
Try serving the fruits in a different way. Whenever I give my birds dandelion leaves, they will only eat it if I weave it through the bars in the cage- if it's on the ground, they ignore it. You could try hanging up the fruit in the cage like you would a toy (make sure to remove it when it starts showing signs of spoiling, such as becoming wrinkled or losing color). You could also try wrapping the leafy green vegetables around small amounts of the fruit you want him to try.
