High vitamin cod liver oil would be good because it has a lot of vit. A in it and the right proportion of vit. D. And you could give sodium ascorbate for vitamin C.
As far as how much and how long...these are supplements I give my kids every day. I give my 2 and 4-year-olds 1/2 teaspoon of clo. and the SA varies, if they are sick, several scoops a day. When they were babies, I let them lick the SA powder off of my finger.
Good luck figuring it all out!