You may have to keep feeding until starts to eat on his own.

12 days is pretty young, and I'm wondering why the mom abandoned him??? Maybe you can block off the nesting area where she moved to and she will return to JR. Usually the mom leaves on about day 20 and dad takes over the feeding chores. Mom starts a new clutch of eggs.
You can feed chick starter feed in paste form to liquid form. Lil one does not drink on his own yet, so the need for feed to be wet. Chick starter is pretty complete nutrition. Easy to make into bottle fed/syringe fed mixture.
Wishing ALL to work out for you,,,