6 month validity on passport

We are planning to travel to Danang in a few weeks.
 We are 2 adults and 2 children (ages 6 & 9)
 I was just about to apply for a visa online but notice that my 9 year  old will only have 3 months left on his passport when we arrive in  Vietnam, is it possible for him to get a visa with only 3 months left on his passport or will he have to get a new passport?
 Thanks,
 Georgina