What would you do

Thought about this last night. Hip displaysia is an issue in some larger breed dogs. It is possible that this is what is happening. One of the test is to move the rear legs to the back and if the go to far they pop out of socket. When he rare's up they could be popping out causing him to fall. This could also explain the same issue when moving quickly to the side. Gitting up can also be harder and more painful when this problem exist. If this is the case all that can be done is mild pain killers mixed with being made as comfortable as possible. It will come down to quality of life in the end but I thought this could explain the issue as u have it now.