Is a husband entitled to compel his wife to make their son address his grandmother as "Mama"? And would the wife be sinful if she refused?

There is no harm in a child calling his grandmother "Umm" (mother), for indeed a grandmother is a mother. There is no objection to addressing someone other than one's biological mother with that term as a sign of respect and honor. However, regarding your husband forcing you to accustom the child to this, he has no right to do so, and you are not sinful for not obeying him, because the obligatory obedience is specific to matters of marriage and its related affairs. Nevertheless, it is advisable not to oppose him, as there is no harm in it, and in obeying him there is mutual affection and harmony.