Okay, first of all, you're not properly keeping the formula between 104 and 110 degrees F at all times while feeding him, and this is the #1 reason that unweaned parrots die while being hand-fed by people who aren't properly educated how to do it. You absolutely must go and buy a candy/cooking thermometer, you can buy them at Walmart for between $10-$20. They are digital, and they have a long, metal probe that you need to keep in the formula at all times while feeding your baby. If the formula is even one degree above 110 F it can burn his crop and cause a wound and infection. If it is even one degree below 104 degrees F, it will absolutely cause his crop to not empty, the formula will basically spoil inside of him, and this is usually the reason that babies die, from crop stasis or a fungal infection in their crops and GI Tracts due to the person simply "heating up" the formula, but not getting it to at least 104 degrees F. This is not optional, and it also will discourage him from not eating it, or eating enough if it's not hot enough...It's a very narrow window of temperature for the formula, but it's the #1 thing you need to ensure you do so you don't kill your bird. You can keep the bowl the formula is in inside another bowl of hot water so that the formula temperature doesn't drop drastically during feeding, however you must ensure that the formula you are feeding him is always betwenn 104-110 degree F.
Secondly, does he have all of his feathers grown-in yet? If not, he should still be living inside of a Brooder that has an ambient temperature around 85 degrees F (if they don't have all their down feathers grown in, then the Brooder must be around 95 degrees F, once all the down feathers are grown-in but the mature, outer feathers are not all in, then the ambient temperature needs to be around 85 degrees F). This again is not at all optional, as the end-result is the same as when the formula temperature is below 104 degrees F; his body cannot properly digest the formula if he's not being kept at the proper ambient temperature, and so the formula just sits inside him and spoils, causing crop stasis and fungal infection. If you don't have a proper Brooder, then you must make one out of a large enough cardboard box or glass aquarium. You must have some type of ambient thermometer located in the back half of the box/aquarium, as the back half will be the "warm" end of the makeshift Brooder. You do this by covering the back half of the box or aquarium with a blanket or towel, and then setting the back half of the box/aquarium on top of an electric heating pad that has an adjustable temperature. (A cardboard box that is large enough works best, as the heat from the heating pad underneath the back-half of the box will penetrate through the cardboard better than an aquarium). So you'll have the back half of the box on a heating pad, and the back-half of the box covered, with the front half uncovered and not on the heating pad. You should use some type of bedding in the entire bottom of the box, as he needs this to prevent the development of splay-leg, and to help to hold his heat in. Simple pine bedding works fine, or better, the paper, "Care-Free/Fresh" bedding you can buy at any Walmart or pet shop is great. Then place some type of ambient or outdoor thermometer in the back half of the box so that you can monitor the temperature and ensure that it is always between 80-85 degrees F. This is the only way to ensure that his body will properly digest his formula.
Other than these two huge issues, that need corrected immediately, you do realize that the older he gets, and the more solid food he eats, the less formula he will eat, right? You should have a very rigid, set feeding schedule based on his age, that changes every week...At his age he should always have a dish of seeds in his Brooder, a dish of fresh veggies/fruits in his Brooder, and actually millet-sprays are probably the best thing to get him started with for eating seeds. He does not need to be fed during the night at his age, he can go 6-8 hours overnight without eating. He should be getting a formula feeding once every 3-4 hours at his age...Now his crop should ALMOST EMPTY between each daytime feeding, but not completely empty. His crop only needs to completely empty once every 24 hours, and that will be overnight. So between his last feeding at night and his first morning feeding his crop will completely empty. Between his daily feedings, his crop should almost be empty...However, if his crop isn't almost empty between daytime feedings, or isn't emptying at all, then this is most likely due to the temperature of his formula being too cool, his ambient temperature being too low, or the formula being too thick.
When you're feeding him, be sure that you are facing him, him in front of you, and you are going into his beak with the syringe on HIS LEFT SIDE OF THE BEAK (your right side if you're facing him), and then you want the tip of the syringe to go OVER HIS TONGUE, and be aimed at a diagonal, across his tongue and pointed at HIS RIGHT SIDE OF THE BACK OF HIS THROAT (your left side if you're facing him). The entrance to his crop is located on the right side of the back of his throat (his right if you're facing him), so that's where you're aiming the syringe.
Never, ever try to force any formula, not ever. If his "feeding-response" stops, then you stop! Otherwise you'll aspirate him. When he's decided he's had enough formula, gently feel his crop. This is a better gauge than looking at a number on the syringe; his crop should be nice and round, like a balloon, but have just a little give to it. It should never be so full that it's tight with no give. The more solid food he eats, the less formula he will eat, and each week you should gradually be reducing the number of formula feedings and the amount in each feeding, OR RATHER HE WILL BE GRADUALLY DOING THIS HIMSELF...This is called "Abundance-Weaning", and it's what you want to have happen. This is why you want to always, at all times, have solid food like seeds, pellets, fresh veggies and fruit, millet sprays, etc. in with him. He will gradually wean himself, not you weaning him...You will not reduce his formula, he will.