Is the husband obligated to make up the time for the other wife (B) if he is forced to travel with wife (A) and her sick son for treatment, or does this obligatory travel not necessitate making it up?

If the wife travels for a necessity that cannot be fulfilled except by her presence, such as accompanying her child, and the motive for travel was not an inclination towards it (i.e., towards travel for pleasure), then you are not obliged to make up for the remaining days. What is permissible for a need is estimated according to its extent.