make sure you use beta carotene supplementation, not vitamin a. you can overdose any animal on vitamin a. if you give him beta carotene, however, his body will convert some of that into whatever vitamin a he needs.
the supplement i've been using is quite frankly a p.i.t.a. to deal with. it comes in veggie "gel" capsule and the beta carotene is suspended in some sort of an oil (beta carotene and vitamin a are both fat soluble). so i don't want to overdo it because of all the fat. the oil suspension is also really orange and colors anything it touches yellow.

anyway, i cut open each capsule, pull the contents up into a 1 ml syringe, and give our bird like 0.2 ml of it daily. i had started him out on a full capsule to try to get things moving along much more quickly knowing you can't overdose on it etc. but now i want to make sure he doesn't gain weight (have been weighing him daily and he's been fine). obviously no one should be doing this for the rest of the bird's life! the key will be in both of us finding beta carotene rich foods our birds love to eat.
anything very orange in color has beta carotene in it. it's fall now, so stock up on sweet potato or pumpkin or carrot and offer steamed/boiled specimens daily (cooking releases more beta carotene). i haven't done a very good job of this, but some birds simply need to be given all their foods in a mash (where you blend the foods all together to keep them from picking out the junk bits

). there are many recipes for mash online (perhaps even here at pf?-- can't hurt to solicit people's recipes here

). you can make a big batch at once, freeze them in ice cube trays, and defrost daily for meals. i think peewee would eat a mash out of desperation. i just don't have a food processor, so i'm a bit limited with the blending part of things for the time being.
goldenfeast also makes this great dehydrated veggie powder that you can bake or blend into other foods your bird will eat. i get mine from the bird safe store:
http://birdsafestore.com/gardenpowder.aspx but it might be available elsewhere. it smells amazing, like a dried salad. chock full of nutrients!
you can also try sprinkling a bit of cayenne pepper onto your bird's foods as that too has vitamin a and makes foods more palatable for them.
another really helpful thing for peewee was sprouting. he was somewhat of a seed junkie coming to us, so we used that against him by sprouting all the seeds he once enjoyed dry. sure enough, he eats his "veggies" this way (as do most birds in the wild)!
lastly, i'm reading up on this diet thing more and more. there's a great book called "what happened to my peanuts?" by gudrun maybaum. i also got an amazing cookbook by the folks i adopted my sennie from, phoenix landing, called "nourish to flourish"
http://phoenixlanding.org/books.html (top book). can't find that elsewhere. lots of different types of recipes, from mashes to birdie breads to treats, all made of healthy natural ingredients with the goal of making it nutritionally balanced. i believe there's also a great dvd by clark and porter (also viewable on that site) that guides you through a lot of this in video.
keep us posted and good luck!