How can he give da’wah to Muslims who do not fast in Ramadaan?

What should be done is to call these Muslims to fast and encourage them to do so, and warn them against being heedless and negligent. This may be done in the following ways: 

1 – Informing them that it is obligatory to fast, and telling them of its great importance in Islam, for it is one of the great pillars upon which Islam is built. 

2 – Reminding them of the immense reward which comes from fasting, as the Prophet (peace and blessings of Allaah be upon him) said: “Whoever fasts Ramadaan out of faith and in the hope of reward, his previous sins will be forgiven.” Narrated by al-Bukhaari, 38; Muslim, 760. 

And he (peace and blessings of Allaah be upon him) said: “Whoever believes in Allaah and His Messenger, and establishes regular prayer and fasts Ramadaan, it will be his due that Allaah will admit him to Paradise, whether he strives in jihad for the sake of Allaah or stays in the land in which he was born.” They said: “O Messenger of Allaah, should we not give these glad tidings to the people?” He said: “In Paradise there are one hundred levels which Allaah has prepared for those who strive in jihad for the sake of Allaah, and the distance between each two levels is like the distance between the heaven and the earth. When you ask of Allaah, ask him for al-Firdaws, for it is in the middle of Paradise, the highest part of Paradise. Above it is the Throne of the Most Merciful and from it spring forth the rivers of Paradise.” Narrated by al-Bukhaari, 7423. 

And he (peace and blessings of Allaah be upon him) said: “Allaah says: ‘Fasting is for Me and I shall reward for it. He gives up his desire and his food and drink for My sake. Fasting is a shield, and the fasting person has two moments of joy, one when he breaks his fast and one when he meets his Lord. And the odour that comes from the mouth of a fasting person is better before Allaah than the fragrance of musk.” Narrated by al-Bukhaari, 7492; Muslim, 1151. 

3 – Warning the one who does not fast, and explaining that this is a major sin. Ibn Khuzaymah (1986) and Ibn Hibbaan (7491) narrated that Abu Umaamah al-Baahili (may Allaah be pleased with him) said: I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “Whilst I was sleeping, two men came to me and took hold of my upper arm, and brought me to a rugged mountain. They said: ‘Climb up.’ I said: ‘I cannot do it.’ They said: ‘We will make it easy for you.’ So I climbed up until I was at the top of the mountain. There I heard loud voices. I said: ‘What are these voices?’ They said: ‘This is the screaming of the people of Hell.’ Then I was taken and I saw people suspended by their ankles, with the corners of their mouths torn and pouring with blood. I said: ‘Who are these?’ They said: ‘These are the people who broke their fast before it was time to do so.’” Classed as saheeh by al-Albaani in Saheeh Mawaarid al-Zam’aan, 1509. 

Al-Albaani (may Allaah have mercy on him) commented: I say: This is the punishment of the one who fasted then deliberately broke his fast before the time for iftaar had come. So how about one who does not fast at all? We ask Allaah to keep us safe and sound in this world and in the Hereafter. 

See also question no. 38747. 

5 – Explaining how easy and straightforward fasting is, and the joy and contentment that there is in it, and how it brings peace of mind, as well as the joy and delight in worship during the days and nights of the fast, by reading Qur’aan and praying qiyaam. 

6 – Inviting them to listen to some lectures and read some books that speak of the fast and its importance and discuss various issues of fasting. 

7 – Not getting bored or tired of calling them and reminding them, speaking in a kind and gentle manner, as well as praying sincerely for them to be guided and forgiven. 

We ask Allaah to make us and you steadfast. 

And Allaah knows best.