Agree with Movingon.
He ought to have discussed it with you - but in the end it is HIS money, his DM left it to him,and trusted him to do what he wanted with it.
Like you, I would be upset that he wanted to give it all away, but I would accept that that was his decision. (Would probably throw it in his face for the next 30 years as the windows rattled and gates came off their hinges.)
I think you have a very kind man there, who feels other people's pain. You aren't badly off, you admit - we would all like more, who wouldn't? but you aren't going to end on the streets. Let him donate it
As Acrossthepond has said - he may well resent you if you revert him doing what he wants. Maybe his mother wanted it to go to charity and told him so, but felt she hadn't got the energy to go through the process of changing her will?
Or perhaps he's worried that once you start spending it, there will always be something else you want - that it will start with (say) paying off mortgage and repairs, goes on to decorating, then once you have decorated the furniture looks shabby so you replace it, then you need a holiday after all the upheaval, and surely it would be best to replace the car before becomes unreliable . . .
It's his money. Leave him alone.