Seeds are not enough. I think this is the likely source of his plucking. It sounds like over the years he has become malnourished.
If he does not like fruit or vegetables at the moment, try other foods. Dry plain cereal or pasta is good. Unflavored yogurt is also good if you can get it. You...
This won't stop the problem long-term but some people buy little "sweaters" for them that prevent feather picking. You could use an old sock with holes cut out of it.