Hi there Maribeth,
Welcome to planDisney and thank you for your question! Additionally, send a big thanks to your son-in-law for his service!
For the
Military Salute promotional tickets, the Important Details section of the ticket page mentions that valid military identification will be required for purchase and use of these tickets by Eligible Service Members. They define "Eligible Service Members" as active or retired members of the US military, including the National Guard, Reservists, the US Coast Guard, the US Space Force, the Commissioned Corps of the Public Health Service (PHS), and the Commissioned Corps of the National Oceanic and Atmospheric Administration (NOAA).
Therefore, it sounds like your son-in-law would no longer be eligible to use these tickets once his enlistment is over. However, I will note that there are typically
various promotions happening throughout the year, so it's possible that he might be eligible for a different one once he's ready to visit the Happiest Place on Earth! Alternatively, if he was able to visit prior to the end of his enlistment then it sounds like he would still fall into that eligible service member category.
Thanks again for your question, Maribeth! I hope you have a wonderful holiday season.
Have a Magical Day,
Greg