I also don't have any personal experience from where to draw a good conclusion on this but, personally, I would stop the syringe feeding. I don't know if I am on the right track or not but I would try it for two or three days just to see what happens... I am just going by the seat of my pants on this, mind you, but it seems to me that the force feedings, medicines, shots, physicals, etc have taken a toll on him and that he no longer trusts that the food you give him is any good. He is hungry but he doesn't want to be fed because feedings have become something to fear instead of something to enjoy. And that is a dangerous thing to have established with a young animal, Chad.
What I would do is forget about nutrition for now and give him anything he wants and would eat on its own. I would eat breakfast, lunch and dinner with him every day (and this needs to be his food and at his schedule so breakfast should be 1 hour after dawn and dinner 1 hour before dusk- so as to take advantage of his natural daily biorhythms) and start by eating something myself and with him just perching in front or next to me. I would talk to him calmly, praise him, sing to him, play with him, etc. but I would not make a single movement toward feeding him. I would put the food on the table so he can reach it but directly in front of me and not him and not in his bowl, either, but a 'human' plate. I would play with the food (break it into little pieces, chew it exaggeratedly, make yummy noises, etc) and watch him out of the corner of my eye to see if he takes the initiative to steal it from my hand or take a bite out of a piece I am holding (you can also try 'fighting' with him with the food -like making 'jabbing' movements with it toward his beak). I would start with something I know he likes (my birds LOVE bread, peanuts, pasta and fresh corn so I would start with that). But, if he doesn't eat anything on its own, I would put the same kind of food I had been eating in his cage and bowl (or wherever his eating place is) and simply walk away. Do NOT handfeed him and do not insist. It has to be HIS choice and not yours. I do not know of a single animal that would commit suicide by starvation and I doubt Robert will be the first in the history of the world. And give it time - at least two days, as long as he is drinking water, he will be OK for a couple of days. The idea is to show him that he won't be forced to eat anything he doesn't want to.