When I do try to wean squabs, I notice millet is pretty much all they'll eat. I can only guess it's due to size. They sometime mouth the peas and corn, but spit it out. The millet they will eat though. You can also experiment with weaning onto grit by flavoring it with anise. The anise oil...
In my experience when a squab somewhat fledges the nest it isn't being "abandoned" as you may think. The cock bird usually feeds squabs sometimes as far out as 5 weeks of age, and I've seen my squeaker aged squabs still go and sit in the nest box as sort of a third wheel for quite some time...