Follow buck long's advice. If the squab is feathered the parent will almost certainly come back to feed it if it is in the general vicinity of the nest.
If very small and downy, it is possible to handrear it but you have to get some special mix that is designed for handrearing pigeons. Young get a substance known as pigeon milk from their parents. This is not milk but broken down proventicular (like a stomach) lining specially grown for that purpose when the parent are incubating the eggs.
A substitute is available from the better pet shops for handrearing pigeons. As the young gets older a different mix is gradually needed, but this is still thin enough to administer with a syringe.
When a baby pigeon feeds it sticks its beak into the parent's beak and the food trickles down into its gullet - you may have seen the same action with penguins etc on TV. You must try to emulate this by holding both sides of the beak with your fingers and apply a slight pressure while the syringe end is inserted in the beak. Pigeons will not gape for food like most other young birds, so they are not easy to rear. Once you have managed to feed it for a couple of days the young will be used to your mode of feeding and it will become a lot easier. When you are successful it is a real satisfying feeling.