Is it Haram for me to talk to my mother's husband's son now that I am married even if we were raised together?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The son of the mother's husband is an alien man so it is not permissible for you to be in seclusion with him and he is not permitted to look at you as a Mahram (unmarriageable) person is permitted. The rule does not change even if you and your mother's husband's son were raised in the same place together.
As for talking to him it is permissible in the limits of necessity, with good manners and without being soft in speech.
Allah knows best.