I don't have any experience weaning the larger birds, so
someone correct me if I'm wrong please =).
I would not wean him on to no seeds. As you said, they have
an important place in the diet, the bad thing is when they
get too much. If you wean your baby onto a varied diet of
fresh food, pellets, and seeds, then that's what he will eat
when he is older. We weaned all our babies like this and
they are wonderful eaters. They will always prefer seed and
eat that first if they have a choice, but they will also eat
just about anything else we put in there too. We weaned them
onto fresh food and soaked pellets first, because they are
softer, then introduced millet. They already knew what
eating on their own was like before they tried the seeds.