I am a fresh Imam in the US. Yesterday an American man declared his conversion to Islam Alhamdulillah. He was addicted to alcohol and had a girlfriend and used to do a lot of haram stuff like that. I see that if I told him right now to stop everything haram he used to do at one shot he may reconsider his conversion to Islam. What is the best approach from the Islamic point of view in this case? Fi Aman Allah!Imam.

Alhamdu-li-llah wa as-Salatu wa as-Salamu ‘ala Rasool-il-llah,
Al-Bukhari reports from ‘Aisha, she said:
“‏ ‏إنما نزل أول ما نزل منه [القرآن] سورة من ‏ ‏المفصل ‏ ‏فيها ذكر الجنة والنار حتى إذا ‏ ‏ثاب ‏ ‏الناس إلى الإسلام نزل الحلال والحرام ولو نزل أول شيء لا تشربوا الخمر لقالوا لا ندع الخمر أبدا ولو نزل لا تزنوا لقالوا لا ندع الزنا أبداً”
“The first part of the Quran to come down was a chapter from al-Mufassal, in which there was a reminder about the paradise and the hell fire, so when the people returned [to their lord] the verses pertaining to the lawful and unlawful were revealed and had it been that the first revelation was don’t drink wine, they would have said: we shall never forego wine, and had it been, don’t commit fornication, they would have said we shall never give up fornication.”
In light of this hadeeth, I would recommend that you don’t overwhelm new Muslims early with details. If you review the hadeeth of the man who didn’t know how to pray and learn from it how the Prophet taught the beginners, you will find simplification for a beginner to be his way.
You should also assign a certain knowledgeable person to mentor that new Muslim, something like a big brother or a big sister. That is likely to cause less confusion for him or her and to be consistent. That is better than multiple people teaching him different things that may be conflicting.
Now, that all doesn’t mean giving that new Muslim misinformation when he asks you. We have no authority to change the deen for any reason.
Allah says,
“But say not – for any false thing that your tongues may put forth,- “This is lawful, and this is forbidden,” so as to ascribe false things to Allah. For those who ascribe false things to Allah, will never prosper.” [Q 16:116]
The sins you mentioned are enormities in Islam and should be given priorities, still with wisdom because he may need the help of a physician for instance to stop his addictions.
So, work to enhance their iman, and tell the truth, but it doesn’t have to be all told at once. Graduality is important.
Allah knows best.