my cousin has drink the milk of his grandmother (but its not sure grand mother denies that by sying there is no milk at that time but she usually kept his breast in his mouth) grandmother's last child was born almost 10 years before my cousin. can he marry with his first cousin or not

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 the situation is as you mentioned in the question that the grandmother denies that your cousin suckled her milk, then there is nothing which implies the prohibition of your male cousin marrying his female cousin, and it is permissible, Allaah willing, for him to marry his cousin unless there is another impediment which prohibits their marriage.