Answer
Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear questioner, we would like to thank you for the great confidence you place in us, and we implore Allah Almighty to help us serve His cause and render our work for His Sake.
Answering your question, Sheikh Ahmad Kutty, a Senior Lecturer and an Islamic Scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
You are not allowed to marry the son of your ex- husband, whether the latter is alive or dead. It is clearly forbidden in the Qur’an. Allah says:
“And do not marry women whom your fathers have previously married – although what is past is past: this, verily, is a shameful deed, and a hateful thing, and an evil way.” (An-Nisaa 4:22)
In conclusion, your husband’s son is a mahram to you; hence you cannot marry him.
Allah Almighty knows best.