He mentioned this while writing the Andrew book but argued that it would be impossible due to lack of sources at this point. But since he has been able to get so much on Andrew, and more is coming out generally, hopefully he'll be able to do a good one.
I expect we will hear about affairs if any did happen, Susan Barrantes for one probably.
But at the same time, I think it'll probably be more like his Mountbatten book : a fair appraisal of Philip, not as negative as the Andrew book had to be (since it's much harder to say what Andrew contributed positively aside from his Falklands service)
i think he will dig into what Philip knew about Mountbatten's potential child abuse, though.
I also think it will ask whether Philip could have disciplined Andrew better. Charles' childhood has already been well covered ofc.
https://www.theguardian.com/uk-news/2026/feb/24/andrew-mountbatten-windsor-biographer-andrew-lownie-entitled#:~:text=Lownie's%20office%2C%20in%20his%20home,accounts%20from%20confidants%20and%20clairvoyants.