It is generally allowed for a male to leave his hair to grow. However, letting the hair grow long is not a sunnah (Arabic for: an action done by Prophet Muhammad and repeated by Muslims in order to gain heavenly rewards) for which a Muslim will be rewarded. Rather, it is the matter of customs and traditions. The Prophet (peace and blessings of Allah be upon him) let his hair grow long and shaved it, and he did not say that there was any reward for growing the hair long, nor is there any sin in shaving it. However, he commanded men to take care of their hair.
However, a man’s hair must not be left to grow long in a way that would resemble women. This is forbidden in Islam as the Prophet (peace and blessings be upon him) said that Almighty Allah would curse men who resemble women and curse women who resemble men.
Dr. SalahSultan, president of the American Center for Islamic Research, Columbus, Ohio, and Member of the European Council for Fatwa and Research, states: There is nothing wrong in growing a long hair provided that the person takes good care of it, for the Prophet (peace and blessings be upon him) is reported to have said, “Whoever has hair, let him take care of it.” It is also reported that the Prophet (peace and blessings be upon him) used to have long hair.
having said that, the general customs of the society where a person lives are to be taken into consideration. It is not recommended that a Muslim contradicts the customs of the society, as long as they are not forbidden. Therefore, if growing a long hair is customarily perceived as a bad habit in a certain society, the Muslim is supposed to respect that even if he prefers to grow his hair long.