He may well get Planning Permission, but that in itself doesn't entitle him to build the thing on the Plan. The Party Wall Act thing will kick it at Building Regulations approval time - the BR people will ask questions about it.
If he wants to come onto your relative's land during the course of the build, he would use the PWA to enable it, the PWA does enable him to remove the fence temporarily to build, then he has to put it back again the same as before.
Bear in mind he has no right to excavate into your relative's land for his foundations - foundations are generally wider than the waal they support.