He was not able to pray before he died; can prayers be offered on his behalf?

Firstly: 

Prayer is the greatest pillar of Islam after the
Shahaadatayn, and it is not waived for anyone so long as he is of sound
mind. Whoever can pray standing must do that, otherwise he should pray
sitting or on his side, or by making gestures, according to what he is able
to do, and Allaah does not burden any soul beyond its scope. That is because
the Prophet (peace and blessings of Allaah be upon him) said: “Pray
standing, and if you cannot, then sitting, and if you cannot, then on your
side.” Narrated by al-Bukhaari (1117). 

The one who prays sitting should lean forward for bowing and
prostrating, making the prostration deeper than the bowing. 

If he is unable to move his head, then should he gesture with
his eyes, or is prayer waived for him, or are the movements waived only and
not the words? The correct view is the third option. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said:
The most correct of these three views is that only the movements are waived,
because that is what he is unable to do. As for the words, they are not
waived in his case, because he is able to say them, and Allaah says
(interpretation of the meaning): 

“So keep your duty to Allaah and fear Him as much as you
can”

[al-Taghaabun 64:16]

So we say: Say takbeer, recite Qur’aan, form the intention to
bow, say takbeer and the tasbeeh of bowing, then form the intention to stand
up, and say, Sami’a Allaahu liman hamidah Rabbana wa laka al-hamd
(Allaah hears those who praise Him, our Lord to You be praise), and so on.
Then form the intention to prostrate and say takbeer and the tasbeeh of
prostration, because this is what is dictated by the shar’i principle,
“So keep your duty to Allaah and fear Him as much as you can” [al-Taghaabun
64:16]. If a person is unable to say the words or do the movements, such
as if a man is paralyzed and cannot speak, what should he do? The answer is
that both the words and the movements are waived in his case, but the
intention remains, so he should form the intention that he is praying, and
form the intention to recite, and form the intention to bow, prostrate,
stand and sit. This is the correct view, because the prayer is words and
movements accompanied by the intention, so if the words and movements are
waived because of one’s inability to do them, there remains the intention.
And if we were to say to this sick person: You do not have to pray, that
might cause him to forget Allaah, because if a day and a night pass without
his praying, he may forget Allaah. So if we make him feel that he has to
pray even if it is only by intention, that is better than saying that he
does not have to pray. End quote from al-Sharh al-Mumti’ (4/469). 

Based on that, if the person asked about reached a stage
where he could not pray in any of the ways mentioned above, then he is
excused and there is no sin on him. 

Whether he was excused or not, if he failed to offer any of
the obligatory prayers, no one should pray on his behalf. This is the view
of the four imams (Abu Haneefah, Maalik, al-Shaafa’i and Ahmad – may Allaah
have mercy on them). 

See: al-Mawsoo’ah al-Fiqhiyyah (2/334). 

The scholars of the Standing Committee for Issuing fatwas
were asked: My mother died after suffering greatly from sickness, may Allaah
have mercy on her. She did not pray for ten days, during which she was
unconscious at some times and awake at others, but she did not pray during
these days. Should I pray on her behalf, or what should I do for her? Is it
permissible to send blessings on the soul of the deceased? 

They replied: It is not permissible to offer prayers on
behalf of the deceased, whether she failed to offer them because of an
excuse or otherwise, or to pray with the intention that the reward for the
prayer should go to the deceased, because that is not prescribed in
sharee’ah. The Prophet (peace and blessings of Allaah be upon him)
said: “Whoever does an action that is not part of this matter of ours will
have it rejected.” Narrated by Muslim in his Saheeh. End quote from
Fataawa al-Lajnah al-Daa’imah (9/65). 

It also says (25/257): If your father lost consciousness when
he became sick and was not aware of anything, then prayer was waived for
him, and he is not accountable when in this state, because accountability
for prayer is connected to reason and he no longer has it. But if he did not
lose consciousness or his reason, but he neglected prayer because he was
unaware that people in his position have to pray according to what they are
able to do, then we hope that Allaah will forgive him and excuse him for his
ignorance of that and for having no one who could explain the shar’i ruling
to him before he died – may Allaah have mercy on him and forgive him.
Whatever the case, it is not permissible for you to offer any prayers on
behalf of your father, because no one can pray on behalf of anyone else. The
basic principle is that prayer cannot be delegated. As for your doing Hajj
and ‘umrah on behalf of your father, that is a way of honouring him, and if
you give charity on his behalf from time to time, and pray for him and ask
for forgiveness for him and uphold ties with his relatives and friends and
treat them kindly, these are ways of honouring your father after his death,
and you will be rewarded greatly in sha Allaah for what you do. End quote. 

To sum up: prayers cannot be offered on behalf of this
deceased, and there is no expiation for the prayers that he missed, but you
should pray for him (du’aa’) and give charity on his behalf. 

And Allaah knows best.