Send mail to wherever the BOP inmate locator currently shows him. That database reflects the most current official record of where an inmate is housed, and it is the most reliable source available to the public. If it shows FCI Pollock, send it to Pollock. If it shows FTC Oklahoma City, send it there.
Federal transfer centers are used to handle mail during transitions. If you send mail to the transfer center while he is still physically there, the facility is experienced at forwarding mail within the BOP system as inmates move to their permanent assignment. Mail sent to the right place with his name and register number will follow him through the system.
The BOP locator at bop.gov is updated as transfers are processed, so checking it regularly as he moves gives you the best current picture. In the meantime, send your letter to whatever location the locator shows today and it will find him.