assur
MB 27:15
There are numerous poskim [e.g., because one will look like a goy (in addition, it is forbidden for a man to have long hair because it makes him look like a woman)].
See RAMBAM HILCHOT AVODA ZARA, CH 11:1. SEE ALSO
SHULCHAN ARUCH YORE DEAH, SIMAN 178.
Even if a person wears Jewish clothing, if his hair is long he transgresses a prohibition of the Torah (!) (SHEELOT UTSHUVOT DIVRE CHAYIM 1, YORE DEAH, SIMAN
30)
The Pri Megadim 27, the Chasam Sofer, the Machatzis Hashekel (27:4), the Kitzur S”A (10:6), the Mishna Berura 27:15, the Ben Ish Chai (Parshas Chayei Sara), and a good number of other poskim mention the fact that long hair (or certain kinds of long hair) prevents the fulfillment of the biblical commandment of donning the tefillin shel rosh.
See Mishna Berura (27:15) that, in addition to problems of chatzitza, a lot of hair can prevent the tefillin from being secured in the right place. Apparently, the Chofetz Chaim means that the tefillin are supposed to be on the head, which may be accomplished even if there is hair in between. It is not supposed to be sitting on a clump of hair, which happens to be supported by the head.
According to kabbalah, a man should keep the hair of his head short (see Writings of the Arizal, Ta’amei Hamitzvot, parashat Kedoshim).
About cutting excess hair – see Zohar, parashat Tazria, page 48b.
See also http://www.theyeshivaworld.com/coffeeroom/topic/shave-head.