Sorry to hear
I don't have personal experience with it but reasons could also be due to cage conditions, diet, humidity or disposition.
Is his cage roomy enough? I read about this sun conure who destroyed his tail in the same manner because his cage was too small so his tail was always dragging along the bars. It probably irritated him. His diet could be making his skin dry and itchy. Perhaps introduce lots of fresh fruits and veggies if he isn't on it already. Low humidity can make the skin dry and itchy for birds too. A good humidifier solves that. Avoid showering him with warm water because that strips him of his feather oils which will also dry up the skin. Lastly, how is your bird in general? Boredom is a primary cause of plucking or over preening. Make sure he has lots of toy variety (light shredding toys he can shred and pick apart, heavy wooden chew toys, a couple or more foraging stations, bells, talking toys, foot toys... For him, shredders are the most helpful. My sun conure loves this colorful shredding toy made of woven strips of rattan. I hang them all over the cage bars and perches, cascading like vines down his cage. He'd just hang on them or hide behind them and shred them as he goes.
Praise him and give lots of happy noises when you see him shredding a toy instead of his tail. Distract him with a shredding toy when you see him messing with his tail. The thing with over preening is, it's a hard habit to break for them once started, so just keep at it and don't be discouraged. My sun never had this problem but its always in the back if my mind, one of my worst fears for him. So I pre-empt and cover all bases by doing all the preventative measures I mentioned above.
Lastly, don't rule out severe medical conditions that only a vet can help you with.
I hope everything will turn out okay did you and your birdy! Let us know how he's doing.
