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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If this son is mature or near puberty, it is impermissible for him to suckle from his mother’s breast because it involves seeing and touching her nakedness (‘Awrah), which is prohibited.
As to a child drinking the mother’s milk from a cup or to the suckling of a young child after the age of two or drinking her milk from a cup, scholars held different opinions about it. It is more cautious to avoid it.
Allaah Knows best.