Well thank god for informed people understanding that charity admin costs pay for vital services - and that if you're working with vulnerable people you need staff to be professionally skilled and trained and supervised. (Presumably its not okay for charities to kill, injure or expose clients to abuse because they were well-meaning amateurs?) Would you want a volunteer surgeon? With his heart in the right place? 100% of Samaritans costs are admin - the phone lines are staffed by volunteers so all the rest of their costs are on buildings, IT, equipment, training, recruitment, management of volunteers, advertising, lighting, heating... Which bit of that would you not want to pay for? Or what about a hospice for your dying parent, would you like the care delivered solely by volunteers? No nurses, no managers, no equipment, no medicine, no furniture - just donated second hand goods...
Anyway. His inheritance is his inheritance. I honestly dont think OP you have a legal right to it. But I would be very very pissed off in your shoes and I think your DH should be prudent and keep a nest egg for the future, as others have said, you just cant know what the future holds, illness, loss of job, disability, alongside education etc. Your DC may want to do further education, start a business or a social enterprise themselves - how great to support those endeavours. And I do agree that 40 percent will go in inheritance tax so the sum may not be as great as imagined.
I think your DH's heart is sort of in the right place, he is still in grief and shock afterall and may not be thinking straight. The grief of losing a mother is so awful. So OP if you can I think you should encourage him to set aside a family nest egg, pay off the mortgage etc, even go on some really educational holidays to give you shared experiences and adventures and open your DCs eyes to the world. But at the same time maybe you could try to think of ways to support his desire to be charitable. You could encourage him to think about not giving money away in all in one lump but crowdfunding new social enterprises, regular donations etc. and get some real pleasure out of giving, even get to see results. Good luck!