can mother make son testify against dad

No. Simple answer is no. There are legal means for wife and husband not to testify against one another. But not children. For instance is the wife forces a child to testify against her husband (say she claims that the husband molested the child) and the event never occurred - and the reason for this force to testify was to get even with the husband or monetary gains through a divorce then court and truth would truly be screwed up. So the simple answer is no.