There is no problem of Chatzitzah 2. Ideally, he should be careful that the Retzuot are tight on the head. 3. One should be careful the knot is above the hollow in the back of the neck.
There might be an issur of Lo yilbash.
The Rishonim argue about a Chatzitzah, a barrier on the head under the Tefillin of the Rosh.
The Rosh (Hilchot Tefillin 18) understood from the Gemara in Zvachim (19a) that a Kohen Gadol had hair peek up of his hat above the Tzitz and there he put the Tefillin because to put on the hat would be considered a barrier between the Tefillin to his head.
The Ran (Brachot 15b) and the Rashba argue if it is permitted to put Tefillin on top of a hat. The prohibition of Chatzitzah is only Tefillin Shel Yad which says the word ‘Os’ but there is no problem by the Shel Rosh. Why didn’t the kohen Gadol put the Tefillin on the Mitznefet, because there is a problem putting another Dvar mitzvah on the Mitznefet of a kohen.
The Shulchan Aruch (Oh 27:4) is stringent like the Rosh and one should not have a Chatzitzah nor by the Shel Yad nor by the Shel Rosh.
The Rema writes only by the actual Tefillin but not by the Retzuot.
Mishna Berura adds the explains the Rema was only lenient by the place of the wrapping the on the arm but any Retzuot which tie the Tefillin the body the is a problem.
It seems there is an issue of Chatzitzah by the Kesher.
R’ Ovadia Yosef writes (Yechaveh Daat 2:2)The is no prohibition of the Chatzitzah with long hair because of Safek Saefeka, two doubts. It’s a Safek if we rule like the Rosh there is Chatzitzah by the Shel Rosh and maybe hair is considered part of the body and it can’t be a Chatzitzah. Even though the Machatzit Hashekel writes that long hair is like putting Tefillin on a hat we’re lenient.
He has to be careful the knot is in the correct place. The Shulchan Aruch (27:10) the knot must be on the back of the head above the neck. The Mishna Berura explains that this means at the end of the skull which is directly opposite the face not opposite the throat ad it is better fit above the hollow.
Therefore, a ponytail which sits high in back will not be able to put the Kesher in the correct place, rather he should put the Kesher under the ponytail on the head.
Another issue with a lot of hair the Mishna Berura brings is the Retzuot themselves will not be tight and one will not fulfil Ukshartem. So, he must be sure it is tight and if it is loose it might fall in out of place .
Whether there is a prohibition of Lo Yilbash, that he looks like a woman:
The Tosefta Bikkurim (4:2) says an Androgynous can marry and can wear clothes and have a haircut like a man .
The Tifreret Yisrael explains that since the Gemara says that women grow long hair like a Lilit (a female Sheid) this Androgynous marries like a man and so can dress like one and get a haircut like one.
So writes the Rambam (Avoda Zara 12:10)-A woman should not adorn themselves like men for example to wear a men’s hat or wear a helmet or get a haircut like a man… a Tumtum or Androgynous should not wear clothes of a woman or get a haircut like a man.
It seems that tis forbidden for a woman to shave or cut her hair short like a man. ( Shach in YOD 182:8)
The Atzei HaLevona asks but we know there are some groups of women who always shave their heads and what a bout a woman who became a Nazir wo shave their heads. It should be forbidden.
He answers the prohibition is only when a woman wants to look like a man Therefore the only problem is if the women would shave her had like a man and leave Peyot but shaving the whole head there is no problem.
In Shu”t Shearit Yisrael writes that there is no prohibition for a man to grow long hair from the fact that the Rambam only says that is a problem for women to cut their hair like men’s in order so the genders should mix together for promiscuity which is only by woman who cuts their hair like e man’s but by a man, since he also has Peyot and a beard ne can tell it is a man and therefore he won’t be able hide among women.
It would seem that today if he shaves and does not have peyot then it would a problem to grow hair.
However, there it is not mentioned anywhere that is forbidden for man to grow long hair.
I have heard some say because Lo Yilbash is only prohibited if one does something but not getting a haircut, is not a problem. However, that might only be an exemption for Malkut.
However, from the Tiferet Yisroel it seems the way of women is to grow their hair long , since an Androgynous can get a haircut like a woman but if he does grow long hair like a woman there might be a prohibition. Tzarech Iyun.