Should he pray behind someone who says that it is not obligatory for women to cover their hair?

Firstly: 

What this man says about America being more of a land of
Islam than the land of the two sacred mosques is false, and there is no
need to produce evidence that it is false, because there is no wise man,
let alone any scholar, who would say that America is a land of Islam, and
America itself makes no such claim. 

Secondly: 

It is obligatory for women to cover their heads according
to scholarly consensus, based on a great deal of evidence which includes
the command to observe hijab, the clearest of which is the words of
Allaah, may He be exalted (interpretation of the meaning): 

“O Prophet! Tell your wives and your daughters and the
women of the believers to draw their cloaks (veils) all over their bodies
(i.e. screen themselves completely except the eyes or one eye to see the
way). That will be better, that they should be known (as free respectable
women) so as not to be annoyed. And Allaah is Ever Oft‑Forgiving, Most
Merciful”

[al-Ahzaab 33:59]

This indicates that it is obligatory to cover the face too,
because the jilbab is lowered from the top of the head until it covers the
face and chest. 

We have explained the evidence for it being obligatory to
cover the face in the answer to question no.
11774, and
this evidence applies even more so to covering the head. 

The scholars differed with regard to covering the face, but
they did not differ concerning the fact that it is obligatory for a woman
to cover her head and hair in front of non-mahram men. 

Imam Abu Bakr al-Jassaas said in his tafseer, Ahkaam
al-Qur’aan (3/485), commenting on the verse (interpretation of the
meaning): “And as for women past childbearing who do not expect
wedlock, it is no sin on them if they discard their (outer) clothing in
such a way as not to show their adornment. But to refrain (i.e. not to
discard their outer clothing) is better for them. And Allaah is
All‑Hearer, All‑Knower” [al-Noor 24:60]:

There is no dispute that the hair of an old woman is ‘awrah
and it is not permissible for a non-mahram man to look at it, just as in
the case of a young woman. If she prays bareheaded, then her prayer is
invalid, just as in the case of a young woman. End quote. 

It should be said to this pseudo-scholar and his ilk: This
hijab or modesty that you speak of: what is the evidence for it, what are
its limits, where does Allaah mention it in His Book or His Messenger
(peace and blessings of Allaah be upon
him) in his Sunnah? If he mentions the verses of hijab, he should be asked
to study their tafseer. If he ignores the verses of hijab, they should be
mentioned to him, “so that those who were to be
destroyed (for their rejecting the Faith) might be destroyed after a clear
evidence, and those who were to live (i.e. believers) might live after a
clear evidence” [al-Anfaal 8:42].

 Thirdly: 

What the Muslims should do is respect the Qur’aan and
Sunnah, and adhere to the limits set by them, strive to learn their
rulings and not accept whatever any charlatan says, especially in these
times when there are so many who have the audacity to interpret sharee’ah
according to their whims and speak without guidance.  

Our Muslim sisters have to fear Allaah. Let their example
be ‘Aa’ishah, Faatimah, Khadeejah and the first generation of righteous
Sahaabi women who hastened to cover their entire bodies when the verses of
hijab were revealed, and when they went out they could not be recognized
because of the black garments they wore, and their faces could not be
seen, let alone their hair. 

Fourthly: 

This speaker should be advised and told of the rulings on
hijab and its nature from the Qur’aan and Sunnah, and the teachings of the
scholars. If he responds, then praise be to Allaah, and this is what we
hope for him. If he persists in his attitude, then he should not be
allowed to teach or exhort the Muslims. He should not be allowed to lead
the Muslims in prayer and they should not pray behind him, rather he
should be shunned and forsaken until he comes back to the truth. 

We ask Allaah, may He be glorified, to guide the misguided
Muslims, and to make us steadfast in following His religion until we meet
Him.  

And Allaah knows best.