Third party counselling is the only answer. He wants to stay for an extra week and keep his son with him. (Who cares about the daughter - not him obviously.) Is this a ploy for him to move back to his home country and keep the boy with him permanently?
I wonder if the husband is afraid that his parents will lose respect for him if he "permits" the vacation to be shorter to accommodate his wife's wishes. He's the man, the boss, the decision maker - is that the way they think and he wishes? Get a third party in to defuse this situation.
And a separate point - DE revelations about his sons' genitals was completely inappropriate. His readers don't need to know and shouldn't know. That was simply disgraceful to mention.