What did Michael's dad whisper in his ear near the end when Dad was lying on the bed in the lounge?

Maybe I need to watch again, but Michael's ill dad said something to him by whispering in his ear near the end. The look on Michael's (Jason Isaacs's) face was enigmatic.

Don't think it was explained in the tv series unless I missed it.

Was this in the book? Perhaps I need to look there, but thought someone might know what i missed?