Never had that happen & I've had 3 open heart surgeries. Has he talked to his doctor about this? It' important to let your doctor know when things don't seem normal. Everyone reacts differently to different things. Could be many things & it could be nothing. Good luck & quick healing!
I never thought about the pillow situation, but after my first OH I had my family purchase a "wedge pillow" - kind of shaped like a long triangle & I started sleeping on that because it raised my head & upper body kind of like a hospital bed does. It worked - my pain eased & my breathing eased. I have used it for each of the 2 OH surgeries I've had since then & it really works wonders. They purchased it at a hospital supply store. That has been so long ago (16 yrs) that I have forgotten how much it cost, but it was worth every penny. I still have it, but hope I don't need it again. Good luck & I hope your dad will be all right.