Letting the hair grow long is not a sunnah for which the Muslim will be rewarded, rather it is the matter of custom and tradition. The Prophet (peace and blessings of Allah be upon him) both 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, but 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 Allah the Almighty cursed men who resemble women and cursed women who resemble men.