You can look for a kid's liquid vitamin C that is sweetened with fruit juices and sugars. Beware of artificially flavoured and sweetened supplements. They are no good for the body and will likely cause worse problems down the road!
Or buy some buffered vitamin c powder and mix a small amount in pure distilled water and administer to the baby with an eye dropper.
Be sure not to give a baby more than 500mg of vitamin C per day, otherwise you may be changing a lot of diapers. The worse that can happen is that baby may reach bowel tolerance which means he/she will be pooping a lot.