There is a known issue at the moment that is preventing Xbox Game Pass users from accessing Sea of Thieves after the patch. The latest list of Xbox Play Anywhere titles can be viewed at /gamepass" Xbox Game Pass currently features a select number of Xbox Play Anywhere games that is continually increasing.
" Can I use Xbox Game Pass on my Windows 10 PC? You do NOT need a Xbox for these games, and you do NOT need to own a Xbox to subscribe to Xbox Game Pass If you have "Xbox Game Pass", you can play any of the titles in the "Play Anywhere" category with your PC. You have the beta version, which is an older pre-release version your son has the newer version, downloaded directly from the Windows Store.There seems to be a lot of ignorant people spreading misinformation on this thread. The different Xbox applications shouldn't make any difference to clarify, both applications are the same. Forgo Game Pass for PC all together, on your son's PC, and hope that they release some sort of sharing bundle, in the future.
Purchase your son his own Game Pass for PC subscription.