Is it obligatory for a man who is about to embrace Islam, and who is married, has another relationship, and has a teenage son, to divorce his wife and live with the other woman, or to leave her, taking into consideration his responsibilities towards his son and his property, and his strained relationship with his wife?

Adultery is a heinous and forbidden crime. You must cut off your relationship with that girl. If you love her, you can marry her, and this is the best remedy to extinguish the fire of desire. The Islamic Sharia (definition) permitted marriage to more than one woman, and Christ Jesus (peace be upon him) brought good tidings of it, and it is the final Sharia. A man can marry up to four women; if he fears he cannot be just, then only one. If it is impossible for you to stay with your first wife, divorcing her is not forbidden for you. Be keen on caring for your son and raising him well.