If you don't want to do anything now, how about (when your DF's fit & well) you tell him you & DH are sorting out your wills and have been advised to set up a Discretionary Trust as a tax saving measure.
How this works is that on the death of the first spouse the survior is left £X (often the savings - they need day-to-day money) and the house (usually the major asset), or the deaceased's share in the house, is left to the children (or whoever), with the surviving spouse getting a life interest in it.
That means the survivor gets to stay living in the house (provides them with stability and security at the most awful time in their life) but on their death it passes, unter the trust, the children.
Ask if he's been given similar advise, ask his opinion, that sort of thing. It might well make him think "that's exactly what I want!".
These sort of trust work wonderfully, because they save tax and mean that the survior doesn't loose their home. And, if the survivor wants to move, they can - they proceeds of the sale of the original home are used to buy another home for them, but they never "really" own it, it belongs to the trust.
Sorry for long message!