In my generation, it was a given that young LDS men would go on a mission. There was a lot of stigma associated with declining to serve. One of my friends was kicked out of his house and nearly disowned for it (his parents came around eventually).
Thankfully, times have really changed, and that outcome is rare nowadays. In my recent experience, most young LDS men are opting to not serve missions. There is a lot that goes into deciding to serve and a lot of them make last-minute decisions one way or the other. There is a lot of indecisiveness and changing plans at all stages of the process.
Similarly, most of the young men I know who do decide to serve come home before two years. That carried absolutely crushing stigma back in my day. That stigma has also lessened considerably over the years (though is still rough).
So rather than missions being a sure thing for every young man for 2 years like they were in my generation, this generation is more flexible with their plans. It’s a really really difficult decision to go and then to stay there for most of the kids I know.
So I sympathize greatly with the contingency plans. I don’t know how OP feels vs the kid in question either. Many times the kids and parents have differing opinions when it comes to serving, and that can be a huge stress for everyone. I hope OP and their kid can make plans that work well for everyone involved.