It is his money to do with as he pleases. That said, he has responsibilities to his family. He appears to consider these covered anyway, and the inheritance is not needed for that purpose.
First, he needs to work out how much will go in Inheritance Tax. Secondly, I also suspect (as others have commented) that his thinking processes are affected by grief. One factor probably not at the centre of his thoughts is that his kids are going to have to pay for their own education past 18. The OP doesn't mention he having a degree, but if he wants his kids to have the possibility of a degree, the inheritance is a heaven sent opportunity to set up FE funds for the kids - which surely he can see his mother would approve of?
The OP mentions his 'Heath Robinson' repairs: to me this suggests someone reluctant to employ tradesmen to do work, rather than DIY from necessity. There are men like that, who want to do it all themselves, however cack handed, rather than employ someone.
I would suggest (as others have) drawing up a list of work that needs doing, and of course the mortgage. I'd not mention fancy holidays as he may be the type who would think spending a windfall on a holiday is sinful, and it would backfire as a suggestion.
Engage with him about his charitable impulse. It is easy to say 'give it to charity' but what does he want to try and achieve? In his chosen area there will likely be many charities, with different approaches, different outcomes, and different cost bases. Some careful research is needed to be sure that his proposed gift does the most good. Getting him thinking positively about this, so his proposed gift will make the most difference, will take time, during which he may emerge more from his (presumed) grief and think more clearly about his own responsibilities. And you can engage positively with his charitable impulse and possibly exert more influence on his thinking than you will by being confrontational, however understandable that might be.
Divorce as many have suggested is a huge step and the financial consequences may not be as positive as many assume. Anyway, surely the relationship is about more than money?