Hope these thoughts have been of some help to you. My S.I. male that is 6 yrs. I get his blood tested every 6 months and his last test was 2 months ago, everything is normal on his tests. He's not sick, eats good, plays all day with his toys, gets 12 hrs. This is not a standard test - so although your bird has had regular blood tests, you might want to ask you vet about this test.
Sprouted sunflower is also nutritious, as is quinoa, brown rice and beans.
It is possible that he may be sensitive to artificially long daylength caused by inside lighting. Be sure to let us know if any of these ideas, or a combination of them, help your boy with his molting difficulties. My S.I. male that is 6 yrs. I get his blood tested every 6 months and his last test was 2 months ago, everything is normal on his tests. He's not sick, eats good, plays all day with his toys, gets 12 hrs.