Of course he should cancel it. But then it should never have been written in the first place. If he cancels it, then what? He lets down a lot of people in the US, where he now lives. If he lets it go ahead, he doesn't risk alienating those who surround him over there.
He may feel like cancelling it while he's in the UK, but back in the US there'll be a LOT of pressure on him not to.
He's also seemingly surrounded by people who advise badly, so I'd be surprised - pleasantly - if he actually did the right thing here.
But it seems awful too that he would agree to cancel it now when he didn't care before about upsetting and/or trashing the work and legacy of the grandmother he is now mourning.
He'd need to be under the wing of The Palace for it to happen and his wife - an independent woman from North America (different culture to royal and aristocratic Britain, NOT related to skin colour or liking her/not liking her) - would be kind of crazy to accept that as it would impact her goes against who she is.
It would be nice if the palace can find an out for him that allows him to save face.