My son has been joining my world for several days without a problem. I am on an iPhone and he is on an iPad, both running latest available Minecraft PE version. We are always on the same wifi network, and I have been logged into my Xbox/Microsoft account but he has not been (we found it to work more easily this way but that's a whole separate story). Suddenly, he can no longer join this particular world. Any other world on my phone, he can join without a problem, but just not this one. The last time the world was closed he was just standing on a standard dirt area in front of our house and now when he tries to join he gets the "locating server" message that will sit there forever; meanwhile, in the world I can actually see his avatar with name tag above it standing there AFK. I have tried blowing up his avatar with TNT (can't hit it with arrows or sword), but nothing affects it. His avatar is wearing a full set of meticulously enchanted netherrite armor so we would really like to save him!!  We have reset the app, restarted the iPad and iPhone, hard reset the iPad — everything short Of deleting and reinstalling Minecraft as we fear this will cause him to lose his items. Oh, and others are still able to join this world perfectly fine (connecting in the same manner with iPhones and iPads). 
thanks!