Abandoned Chick

Hi Everyone,
I am an unexpected and inexperienced parent a chick. We live on an island with wild/feral chickens and a few days ago my son found one in the yard. I left it there for several hours hoping the mama would hear it's peeps but she never came. So we adopted him, however there is not really a way to get chicken starter so I have been feeding it a mixture of quinoa, steel cut oats, instant oats, brown rice, and flax seed all ground up in addition to whatever small bugs and ants we find outside. Is this ok, is there anything else I should be doing? Its feathers are coming in but it doesnt seem to be getting any bigger. Thanks for any help.

 

Thank you for your suggestions. I let my son take it outside with him, it just follows my son like a puppy, so i think its eating enough dirt/gravel. Im trying to treat it as if it was still with its mother as much as possible.