Unfortunately the federal Bureau of Prisons does not have to tell the outside public anything that they are doing relating to inmate punishment or transfer. If your inmate is in the SHU, he is allowed one 15-minute phone call per week. He is allowed to receive mail and he is allowed to write. If you have spoken to your inmate's counselor or case manager (or unit team secretary) before that might open the lines of communication, but it is not something they often do.
"In the SHU pending transfer" tells us that he is in a disciplinary situation that requires his transfer. Depending on the offense, the staff might not be willing to share the new charges with you. They will rarely let you know where or when he is being transferred for safety-sake.