All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The suckling that prohibits marriage is the suckling that took place within the age of two years old according to the most preponderant opinion of the jurists .
Therefore, suckling this child who is two and a half year old does not make him Mahram; for more benefit, please refer to Fatwa 86312.
Hence, when this child reaches the age of puberty, this woman must wear Hijab in his presence.
It should be noted that care and kindness (toward a child) is something required and an act by which one gets closer to Allaah; as regards adoption, it is forbidden. Therefore, if this child is traced back to other than his father, then it is an obligation to rectify the matter and trace him back to his father. For more benefit, please refer to Fataawa 88415 and 86418.
Allaah Knows best.