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. We ask Allaah to exalt his mention as well as that of his family and all his companions.
It is permissible for this man to marry you without informing his family about it provided this marriage meets the requirements of a proper marriage including the presence of the guardian of the bride and two witnesses. Since this man is sane, he does not have to inform his parents or others of his family about it. However, it is better to inform them if there is no great benefit to be achieved by keeping it a secret for a while.
Allaah knows best.