I would like to ask if a marriage between a man and a woman will be valid even without any witness from the man's side (his family doesn't know) but the girl's parents gave their consent for her to marry him.
Is it only the girl who should get the consent of her family to marry or the boy also have to get the consent of his parents? What if the boy's parents are against the girl? Can he still marry her providing she is a Muslim?

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.

Parents have great rights upon their children as we mentioned in our Fatwa: 83393.

So, it is better to convince the parents using all possible methods to accept his marriage with a girl he likes. If they do not agree with him then it is better to respect their opinion due to their great status granted by Allaah, the Glorified and the Exalted. Parents do not have the right to force their children to marry someone against their will as we mentioned in our Fatwa: 83008.

However, the validity of the marriage of a woman depends on the presence of her guardian or his deputy while the matter is not so regarding a man. A man does not need a guardian's approval for his marriage to be valid.

Allaah knows best.