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.