What is the ruling on a man entering upon the son of his father through رضاعة (nursing) from his father's second wife?

If the second wife breastfed the person in question with your father's milk, then he becomes your brother through rada'ah (milk-kinship). It is permissible for him to see what a man may see of his maharim, such as the head, neck, palms, and feet. Solitude and handshakes between you are permissible without suspicion. It is stipulated that the suckling that establishes prohibition must consist of five full, satisfying sucklings within the first two years of the infant's life.