@NiceShrubbery
Surely they wouldn't allow "His Grace". Headline writers would have a field day (Dis-)
They have to, if he retained his dukedom. It's the normal form of address for a duke. Perhaps that's why they're not in a hurry to do that.
Much better, perhaps, to do nothing about his personal styles and titles, and just quietly and inexorablely remove his military ranks and patronages, and leave him in a form of 'exile' at Windsor, appearing only rarely at family occasions. Which essentially means births, marriages and deaths (but christenings often private anyhow, and he can be let in the crowd for marriages and many funerals).
Jubilee occasions will be a bit trickier - but as today's BBC article is foreshadowing even the queen might not be present at as much as originally planned, then it might be possible to lose him from everything except perhaps any formal service of thanksgiving