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.
You have to know that it is not permissible to organize a ceremony in which men and women mix. In most cases such mixed celebrations do not observe religious etiquettes; therefore, this is a sin which causes much harm. As a guardian, you are responsible if this mixed banquet is held. The Prophet said: "The man is in charge for his family and is held accountable for them." [Al-Bukhari and Muslim] Therefore, we advise you to fear Allaah and do not let this celebration take place under your guardianship and it is not permissible for your son or anyone else to attend it as long as it is mixed. The fact that your son is unwilling to attend this marriage banquet is not considered as being undutiful to his father. On the contrary, you should be happy with a son like this and help him in performing good deeds, and not be angry with him for that reason.
Allaah knows best.