Should he obey his parents or follow the Sunnah?

Praise be to Allah.

He should try to convince his parents and explain to them the Sunnah of dressing, eating, sleeping, etc. If both or one of them still persist, then he can obey them with regard to matters of dress, eating and sleeping that do not go against any text or clear evidence (daleel), because the basic principle regarding clothing etc. is that it is permissible (unless there is evidence to the contrary) and it is usually based on custom, because these are matters of nature or tradition which the Prophet (peace and blessings of Allaah be upon him) and others used to do before Islam. So there is nothing wrong with a Muslim wearing the clothes that people in his country wear, or eating the food that they eat, or going along with them in matters which do not go against any clear text, especially if his parents ask him to do that. 

Shaykh Ibn Jibreen 

But if they tell him to do something which goes against the sharee’ah with regard to clothing, for example, such as letting one’s garments hang below the ankles, or they tell him to eat with his left hand, for example, then he should not obey them in that, but he should still be kind towards them.And Allaah knows best.