You absolutely want him to eat a large variety of foods. This is how you wean him, it's called "Abundance Weaning", and it's absolutely the most healthy way to wean a baby bird. Absolutely, he can have raw apple, but a lot of fruit in general is not good for any bird, baby or adult, as it is full of sugar. But an apple slice or two a day (small apple slice) is not a bad thing. It sounds like he is starting to wean himself now, and that's why he is only eating a small amount of formula per feeding (though I'm assuming you are keeping the formula at the proper temperature, between 105-110 degrees Fahrenheit throughout the entire feeding, because if the formula temperature drops below 105 degrees he may reject it, keep the formula bowl inside another bowl full of hot water, and keep the thermometer inside the formula the entire time to monitor the temperature, this will keep the formula warm throughout the feeding).
Now that he is starting to take less formula per feeding you need to provide him with all of the foods you are going to wean him onto and the foods you want him to eat as an adult. So you always want him to have access to fresh, clean water at all times, a small, high quality, natural pellet like Zupreem, Harrison's, or Roudybush, an assortment of fresh veggies (this is very important, you want him to learn to like fresh veggies), and millet sprays so that he can learn to crack and eat seeds. You may need to soak his pellets in water or fruit juice to soften them before he'll eat them, then gradually he'll eat dry pellets. The apple can be given as a small treat, but you want to let him try a different fruit each day, not just an apple slice. As I already stated the small piece of fruit is only a once a day thing as it has too much sugar in it, but it can be raw, no need to cook it. The pellets, a variety of healthy veggies, and the millet spray (after he learns to eat the millet spray you'll want to give a high quality loose seed mix for small hookbills, as this should be a supplement to his staple diet of pellets. The idea right now is to provide him the pellets, a variety of healthy veggies, and seeds daily right now because he needs to learn how to eat them and to like them. The fruit is only a once a day, small treat or a small part of chop mixes if you're going that route.
So you'll need to offer him his normal feedings each day just as you have been, and as you've noticed he will gradually decrease the amount of formula he eats during each feeding. So feed him his formula first thing in the morning as usual, as much as he will eat, and then give him his pellets and a millet spray and leave them in his cage. He'll probably play with them at first (if he's not already eating pellets or seeds) and again you may need to soak the natural pellets in fruit juice to entice him to eat them (the colored, fruity pellets are full of sugar too and not nearly as healthy as the brown, natural pellets). Once you see him eating the soaked pellets you should start putting some dry pellets in with the soaked ones and gradually phase the soaking out. Then feed him his afternoon formula, as much as he will eat, then again make sure he has pellets and a millet spray or seeds and give him some fresh veggies to try. Take note of what he likes. Then give him his evening formula feeding, as much as he'll eat, then maybe give him his piece of fruit. I always left a millet spray and fresh pellets in the weaning cage overnight, that's up to you, but if you're soaking the pellets you must make sure to change them out and clean the bowl at each feeding time to avoid bacterial growth! This is important.
He will gradually eat more food and less formula at each feeding, but you must keep offering each formula feeding until he absolutely totally refuses an entire feeding. Usually it's the mid-afternoon formula feeding that they refuse first because they are eating more pellets and millet during the day. So when he completely refuses a formula feeding that's fine, just make sure he has pellets and millet/seeds and fresh veggies instead. Once a baby first refused a feeding I would always still try to offer it the next day, and if he refused it again then I would just give him the pellets, seeds, and veggies instead from that point on. Usually the morning feeding is the next to go, but they tend to hang on to the night feeding the longest. Once he is eating his pellets, seeds, and fresh veggies throughout the day he will eventually refuse all formula feedings. However, he may start begging, whining, and head bobbing for a "comfort feeding" of formula, usually at night right before bed. Make sure you give him this, as much as he wants, because it's an important part of the end of the weaning process. Once he has refused all feedings each day, completely stopped begging for any comfort feedings, and been regularly eating his pellets, seeds, and fresh veggies every day for a week or two, he's completely weaned and no longer needs any formula.
"Dance like nobody's watching..."
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