Should he obey his father and buy him alcohol?

Allaah has enjoined upon sons to honour and
obey their parents. Allaah says (interpretation of the meaning): 

“Say (O Muhammad): ‘Come, I will recite
what your Lord has prohibited you from: Join not anything in worship with Him; be good and dutiful to your parents; kill not your children because
of poverty’ — We provide sustenance for you and for them. Come not near to Al-Fawaahish (shameful sins and illegal sexual intercourse) whether
committed openly or secretly; and kill not anyone whom Allaah has forbidden, except for a just cause (according to Islamic law). This He has
commanded you that you may understand”

[al-An’aam 6:151]

And Allaah says (interpretation of the
meaning): 

“And your Lord has decreed that you
worship none but Him. And that you be dutiful to your parents. If one of them or both of them attain old age in your life, say not to them a word
of disrespect, nor shout at them but address them in terms of honour”

[al-Isra’ 17:23]

This obedience is obligatory, unless they
tell you to commit shirk (associate others in worship with Allaah) or to commit sin. 

Because there is no obedience to any created
being if it involves disobedience towards the Creator. 

Alcohol is forbidden according to the Qur’aan
and Sunnah and scholarly consensus. 

Allaah says (interpretation of the meaning): 

“O you who believe! Intoxicants (all kinds
of alcoholic drinks), and gambling, and Al‑Ansaab (stone altars for sacrifices to idols, jinn etc), and Al‑Azlaam (arrows for seeking luck or
decision) are an abomination of Shaytaan’s (Satan’s) handiwork. So avoid (strictly all) that (abomination) in order that you may be successful.

91. Shaytaan (Satan) wants only to excite
enmity and hatred between you with intoxicants (alcoholic drinks) and gambling, and hinder you from the remembrance of Allaah and from As‑Salaah
(the prayer). So, will you not then abstain?”

[al-Maa’idah 5:90-91]

Ten people have been cursed with regard to
alcohol, including the one who buys it. 

It was narrated that Anas ibn Maalik said:
The Messenger of Allaah (peace and blessings of Allaah be upon him) cursed ten with regard to alcohol: the one who squeezes it (the grapes
etc), the one for whom it is squeezed, the one who drinks it, the one who carries it, the one to whom it is carried, the one who pours it, the one
who sells it, the one who consumes its price, the one who buys it and the one for whom it is bought.” 

Narrated by al-Tirmidhi, 1259; Ibn Maajah,
3381. 

This hadeeth was classed as saheeh by Shaykh
al-Albaani in Saheeh al-Tirmidhi, no. 1041. 

To sum up, it is not permissible for you to buy alcohol for your
father, or to obey any created being if it involves disobedience towards Allaah, even if that will make him angry and make him pray against you,
because he is sinning by doing that, and his du’aa’ carries no weight in sharee’ah. 

It was narrated from ‘Aa’ishah (may Allaah be
pleased with her) that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever pleases Allaah by angering the
people, Allaah will suffice him, but whoever angers Allaah by pleasing the people, Allaah will leave him to the people.” Narrated by Ibn Hibbaan
in his Saheeh, 1/115; classed as saheeh by al-Albaani in al-Silsilah al-Saheehah, 2311. 

We ask Allaah to guide your father. 

And Allaah knows best.