Could you please tell me who was Al-Hussein ibn Salem?
Was he a Jew?
How did he convert to Islam?
Where did he live etc..
Is he mentioned in Bukhari?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Such a person is not mentioned among those men from whom al-Bukhari reported the Hadith in his book 'al-Sahih'. Also, we did not find this name in other books of biography.
But, if you mean Abu al-Hasan Ibn Salam there is no any indication that he deserted Judaism and embraced Islam.
Allah knows best.