There is very little you can do from outside where the BOP is concerned. If his request for adminstrative segregation has been denied then he will need to continue filing paperwork. The next filing is a BP-9 and then a BP-10. If the staff does not see signs of physical abuse, then that is why they are not "believing his story". Inmates labeled as confidential informatants are not normally "threatened", they are usually beaten up to where it is pretty obvious to the staff there was an incident. Bad paperwork means that the inmate didn't follow procedure.
We don't know enough about his individual situation to give you more information but if this is something that is really serious you can appeal to the Congressman in your area to file an inquiry from their office to the BOP Adminstration in Washington DC.
Accepted Answer Date Created: August 08,2015