Fruits and vegetables are great that is fine. You do still want to keep some seed in his diet about 10 to 20 percent.
I rarely feed pellets anymore cause of all the additives and most don't really care for them when they have fresh food in the cage.
It took me 6 months to get my Que off an all see diet. It might take even longer with an older bird.
BTW right now let him have all the seed he wants sounds like he is hungry and a hungry bird rarely tries new things.
One of the things that helped is offering foods in different ways. On a stick, masked, diced, cubed, chunked, hot, warm, cold ect....
Also try eating stuff in front of him and make lots of yummy sounds. When he tries to get it play keep away until he is almost standing on his head to get you to let him have some then let him have it. This works well with Que and getting her to try new fruits and such.
Good luck