DSTC, thanks for the info. difficult child has been saying he was going to join one branch or another for several years now, usually when something else is not going well, and he doesn't follow through. I am not realistically expecting him to lie (or sign), just happy that he was thinking about the future and trying to encourage that.
On the other hand, if I really want to increase the chances of him enlisting, I will be sure to tell him all of the above reasons why he shouldn't.