if its happening everytime you approach, it could be something in how youre approaching him. especially since you have other birds and are comfortable with their behaviour and have a bond with them, it may be a subtle difference in his nature that you are accidentally overlooking.
i recently took in salvador's sister, queenie, for several weeks as my friend was having alot of difficulty with her. when they approached her cage, shed get very scared and hide at the back, or jump around in a panic. she wouldnt even take treats through the cage. once they had reached in and grabbed her out (though clearly this wasnt enjoyable for her), she would be perfectly fine sitting on their shoulder - infact shed make a mad dash for the shoulder. but if a hand came anywhere near, shed freak out.
they have had quite a few other ringnecks, and their experience with other ringnecks and how they interacted with them was preventing them from realising the difference in their nature - the same with people, you cant teach all children the same lessons in the same way. it was the first one my friend had raised herself, and she was at a loss, to the point she was thinking of selling her or giving her a new home.
the first thing i did was stopped grabbing her. i believed that was the number one cause of her fear of hands. probably not the best thing, but salvador still likes a little fruit puree in the mornings and night, off the spoon like when he was a baby. so i tried this with her aswell. the moment my hand approached, even with food, she freaked. so i backed off and gave her space. i placed her on a t perch about a metre away and gave salvador his food. well, he gobbled down twice as much as usual with the presence of another bird, but by the end her curiousity got the better of her and queenie tasted the smallest amount of puree and then back right off again. within a few days she was taking more and trusting alot more.
i would also spend atleast 2 half hour sessions with her each day (if you dont have this much time, offer what you do have). atfirst id just sit infront of the cage and rtalk to her, and hold a little treat in - not really offering it to her, but jsut so she got used to my presence. everytime i went passed the cage, id also drop a small treat into her food ball - really obviously so she was watching. after a few days, i would open the cage door and sit and talk. i started reaching a little further in with the treat (something easy like millet spray that they can grab without having to come super close).
it was these small wins that started to gain her trust. i tried reaching in the cage and offering my finger for her to jump onto rather than grab her out.also when she was perched on my hand, i dropped my elbow so she couldnt get to my shoulder - the hand was the highest point, and therefore the safest. i also gave her lots of praise and treats. suddenly my hand became the happy place, not the shoulder.
im a bit upset, as she went back to her proper home yesterday, but my friend has been amazed at the difference in queenie each time she came around. i also explained to her what id done, and why id done it. sometimes its the people that need training, not just the bird
sorry for such a lengthy response, but your situation sound almost identical to the one queenie was in, so i thought id explain, and then it all just came out. i hope it help. sometimes, i think, its best to work within the boundaries the bird sets, rather than the ones we establish for them.
good luck!
my poor fingers. my main issue is time mostly. not much time for bonding, so i actually think i must be stressing him... he doesnt bite me anymore and lets me aproach more everyday. but i like to abuse him, petting him and penut butter o.O an advise on another post. 