Breathe...Sorry this is long but...
A collar isn't going to fix the problem unless you plan for him to wear it for the rest of his life. His plucking is something that was a formed habit prior to you getting him. A physical block will not fix it, it is not a physical problem - it is an emotional one. This is how he has learned to deal with his feelings. Its not
YOUR fault. You took on a known plucker. Even when you provide the best environment possible, he still may pluck (and will more than likely continue to do so for the rest of his life). When I first got Romeo (who was a known shredder prior to me getting her) I thought that as long as I provided the best environment it would stop immediately. This is a common but
FALSE way of thinking. She went 5 weeks without touching her feathers, then one day I came home and she had completely shredded her chest. I cant even begin to tell you what kind of perfect environment she is in (shes beyond spoiled - I am her servant :52

. I was devastated and thought I did something wrong. When in actuality there is
NOTHING you can really do to fix the problem. He was probably leaving his feathers alone because he was in a new home with very new distractions - He has now settled in a bit and has decided to be himself. Over time, as long as you keep providing the most stable best environment possible, his plucking will curb some but
NEVER truly go away. Look at it like an
ADDICTION, he did not get to this state overnight - it cannot be cured overnight either. This is a life long problem, just when you think he is done - he will relapse. You are living with an addict (in a bird form) don't take what he does personal, just love him and help him through it. One of my vet's first words to me were 'you will never fix this, either be prepared to live with this problem for the rest of the birds life or walk away now' - my love for this bird and my connection with her, I could never leave her just because shes not perfect. You will eventually get to a point where he may go weeks without incident, then a few months, then a few years, then out of no where when he looks absolutely perfect he does it again. Ignore the problem and act like its not there, don't give any attention when he does it. If you come home and hes plucked himself bald don't treat him any different than normal - act as if you don't even notice it. Enjoy him for him and forget the feathers. This is an emotionally unstable bird and its going to take
TIME, LOTS OF IT. Go in with the mindset that hes yours for the rest of his life and if he wants to be naked then so be it. Trying to physically block the problem (collar) is going to cause him to become even more emotionally upset as he has no outlet for his pent up feelings, plucking is his way to cope. Worse would be if he tries to find a different outlet - then you will have two behavioral problems instead of one. Provide the perfect environment, accept his condition, and you will be surprised at the results. Its a very long hard road but remember that this is what you signed up for. You saw he needed help and you came to his aid. You could of easily bought a baby from a breeder and never had to deal with this problem. What you did do though is take on a wonderful bird that most would of walked away from, due to his problem, and are giving him the chance to really enjoy life. Rescuing is not easy, but extremely rewarding in ways you would never had known had he been perfect from the start. Stay strong and you will eventually get through this.