Pain medications suppress the central nervous system, slowing the cardiovascular system affecting circulation and causing cold extremities. Makes sense that its not an issue when he's moving. Had the same issue myself. If his hands or feet turn blue and he's using fentanyl, get him to the ER as its a sign of an overdose. Almost lost me twice that way. Subsequently, if he's on the fentanyl patch make sure its in an area that he doesn't lay on or gets hot (shower etc) as they are body heat activated.

Eric