I am one of those who has claimed a Budgie like Cockatiels, can live a fairly good long life on just seeds.
And evidence is starting to emerge that they would be better on just seeds than just pellets. There are some reasons for this that I will go into here.
Not that this is what one should do!
They do need a good variety of fresh natural foods as well.
You could try sprouting. I prefer Mung Beans. But any fresh seed can be served as soak seed or sprouted.
Dark green leaf veges are all good. Dandelions, Sow thistle, are two good weeds for them.
One of the things that did come up in talking about seed diet,is. Does he just pick out one type of seed? That is a hard one to change.
Getting picky eaters is also a hard thing to change.
I am glad to say I do not have this problem here.
One recommendation seems to be. Fresh foods in the morning and seeds in the pm.
When changing diet you do need to make sure they are eating enough, and not just waiting til you put the seeds in.