Do you really think that he understands the consequences of destroying the will?
Do you think he is
not being "encouraged" to do this by his girlfriend (who I suspect wants the will to be made in her favour)?
If the answer to these questions is "no" then I dont believe that he has the capacity to change his will.
In this case -delay, delay, delay, deflect, use "love lies" and delay again. He doesnt actually have to have his previous will to make a new one, so there is actually nothing stopping him making another one, but I suspect that he is no longer able to follow this through.
You have the POA arriving soon and there is no legal reason for you to stand down - just put it to one side for a while until it become more obvious that he is impaired. I think that if you stand down you will regret it later. Really, it is up to him to revoke the old one and put another one in place, but again, I suspect that he cannot follow this through.
I wouldnt actually enlighten him about making a new will or POA. You do not have to make it easier for him to do this.
BTW, having gone through the Court of Protection to gain deputyship for mum, and also living with a man who has all the symptoms of FTD, but no diagnosis, I do not feel that the course of action to a[[ly for deputyship suggested by
@MartinWL would be appropriate ATM. I do not think you would be able to get a health professional to fill in the form saying that he has lost capacity which is required with the application. If you stood down from the POA then eventually you might have to do this, but I tell you - it is a whole load of hassle, takes a good six months to go through the court, plus you already have the POA.