Does the child to whom you donated blood at birth become your son through rada'a (nursing/fosterage), and consequently, does this prohibit your daughters from marrying him?

The child does not become your son through fosterage by means of blood transfusion, and it is not forbidden for him to marry your daughters. This is because the rulings of fosterage are related to the suckling child being nourished by the milk of the wet nurse. It is not correct to analogize blood to milk, for blood is not nourishing in itself, whereas milk is fundamentally a source of nourishment.