I would guess that he either has an underlying health condition or you missed the weaning window which is the normal time a younger chick would wean itself. It tends to happen with people that chase the baby around trying to get it to eat formula when the baby is actually trying to refuse food.
The first thing I would do is get a full work up done at the vet. Blood work and culture and sensitivity. Also, make sure you get a gram scale that weighs in grams and is accurate up to the full weight of an african grey. Before doing anything, weigh your bird every morning before you feed him and record that number. Weigh him again at some point later in the evening. The most accurate weight is the one first thing in the morning when he's empty, but the evening weight should give you an idea of normal variation.
If the vet results come back clear of health issues, you may have to force wean him since he's now learned to be dependent. While you are at the vet, ask the vet to assess his weight. If you have to force wean him, accept that he will loose weight. Babies tend to loose 10% or more of their weight before they start flying. They tend to do this by refusing to hand-feed. If an inexperienced hand-feeder persuades the baby to eat formula anyway, they can end up in the situation you are in now. If the vet says he is already super thin, disregard this and follow your vet's advice. If his weight is okay, or at least not dangerously thin, I'd start out by transitioning him to a bowl and/or spoon. No more syringe feeding. Also, soft cooked warm veggies are a good transition. I do a lot of cooked and mashed sweet potato mixed with cooked soft rice, quinoa and beans. Plus a very small amount of banana mashed together. See if you can substitute the veggie mash for one or part of his hand-feeding meals. Once you get him eating off the spoon, try just putting the spoon in the bowl and see if he will eat from the bowl.
Another trick is to give only partial handfeedings and then immediately put him in his cage with fresh food. Let him see you put new and interesting food in his bowl. Give him half a hand-feeding, then directly back in the cage. No play time, no cuddles. Put in some fresh veggies, the mash I mentioned or just some seed and pellets and then you walk away. Do NOT stand there and try to persuade him to eat. Leave so he can't see you and don't come back for a few hours. If he can't see you to fixate upon begging and he's got a good start on breakfast from hand-feeding so he's not panicking over food but he's not full either, he should be naturally curious to try whatever is in his bowl. It helps if the stuff in his bowl is also warm and mushy but hopefully not formula and is some variation of adult bird food. Some people also find it helps to eat something with the bird. So if you try that, instead of leaving, fix yourself some people food and eat it while continuing to ignore the bird. Birds like to eat as part of a social behavior, but if you are giving him attention or looking at him, he may stop eating and try to beg.
Accept that he may loose some weight. But monitor it. If you think its getting to be too much, ask the vet. In all honestly, it might be about the point that you think its getting to be too much weight lost or even a little beyond that point that he starts eating on his own. I'd start with feeding half his meals with hand-feeding and expect that he will eat at least a little bit of regular food on his own. It may very well take about 3-5 days of getting half rations of formula before he's hungry enough to try eating on his own. If he's starting at a healthy weight, he will lose weight but will probably be okay. If he's already terribly thin, you will have to proceed with more caution.
