The physical therapy is done to restore range of motion, “walking normally” and to alleviate stiffness in the muscles.
Physical exercise in the form of walking helps and is necessary to restore a normal walk and motion, but it has to be balanced with the bones healing (i.e. consolidating).
It depends from person to person, but normally one just stays in contact with the doctor from the place where the procedure was done. They know the patient best and what is the best course forward.
No one here knows your nephews case exactly, as this would be impossible for us, but based on the information you have provided, it sounds normal. This procedure requires patience and calm. The last thing your nephew needs is the stress of his family being worried and talking to everyone about his case, which I guess is why he is sparse on sharing information.
It takes 1-2 years to heal, as he adapts to the new height, gaining more range of motion as time progresses. The best advice would just be to tell him that if he needs anything you are there for him, and otherwise just give him time and space.
You don’t want him trying to run and jump around ahead of time to try and alleviate family worries. Just accept that he needs time and patience, and all you have to do is tell him that you are here for him, if he needs it, and otherwise trust that him and his doctor know the current information, and are acting accordingly.