HE can't get carrier updates at all. Not until he is back on Verizon network. He can update applications over wifi, when he can find a connection.
Some phones can receive, download and install system updates over the wifi only connection without being connected to the Verizon Wireless network. There is no guarantee that the DROID Turbo is one of those models until the customer tries to update over wifi only.
The only reason I suggest it is possible is I have updated one of my devices when it was not active on a line, without a SIM card and on airplane mode over a wifi only connection in the USA.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
You do not say whether your son is overseas Military or Civilian. Makes a huge difference. If civilian then I suggest he get a local country phone that works in his area. And you as parents should be communicating with him by Skype over the internet. It is free.
These phones are not huge investments. And they are only intended to be used about 3 years until replaced.
If he is military, then he gets occasional access by portable military provided hotspots. Probably the most state of the art
and controlled networks there are.