I’m sorry but he has Stage 4 Cancer and there is NO CURE. Also in the US there is a thing called the FMLA, Family Medical Leave Act, specially designed for this type of situation.
The sons should take time off of work and take their father to Pakistan, while the rest of the family stays behind. They should then consult the local hospitals in the Pakistan and decide the best possible care so that he is not in pain during his last days.
Oh and yes, I would leave everything behind to fulfill the last wishes of my parents.