Even with the best possible intentions, in your DF's situation it is virtually impossible to treat two DCs in a way that they will both think they have been treated fairly. We have a very simiar problem and still have not worked out how to deal with it, and I am posting on your thread hoping to get the sort of helpful responses you have received. I hope you don't mind.
We have given DS1 £150,000 for house purchase. This was in two lumps, £50,000 for his first house, the rest when he moved up the ladder.
DS2 has been given £175,000 for two house moves, £75,000 of this was spent doing up and extending his current house, with me project managing as he is local, at work, and at the time without a partner. So on paper he has been treated more generously.
But it is he who feels hard done by, because DS1 now has a house in a nice part of London, albeit a small terrace, currently worth well over £1 million, and going up thousands every month, whereas DS2 lives locally, and though his house is twice the size of his brother's its value has not increased in anything like the same dramatic way.
He feels we should leave him an extra amount in our will, to compensate for this disparity. We feel, as of course does childless DS1, that DS2 has chosen to live locally and have three children and we cannot be responsible for his choices.
All this is quite openly discussed, our sons are not grasping (though no doubt some on here will say they are) . At the moment DH and I are more or less decided that equal shares in the will is the only way to go, but I do hate the notion that either of them might feel less loved than the other.
Any opinions welcome, and, OP, I understand how you feel.