This situation must be very frustrating for a foster parent.

There was a topic on this in the archives. I think it was Barb who suggested that it is possible to draw out the vaccine using a poultice as soon as possible after the injection.
Obviously that is not ideal, since mostly it goes into the bloodstream but it's better than the whole lot going in.

Sometimes sulfur, sometimes silica, sometimes thuja, and other remedies - it depends on the child. If he is in perfect health at the moment then there is not much to do homoeopathically, other than to find a good local homoeopath to whom he could be taken should post-vaccine symptoms arise.
If there are a few things about his current health that are not quite ideal then you might take him for a remedy a few weeks prior to the injections, so that he is in optimum health for them, and is therefore in a condition less susceptible to their effects.