I was a caregiver, and the closer of my two brothers began taking a weekend once a month for me (I was very fully employed, and we had a live-in attendant for weekdays). There is a lot to do that an attendant doesn’t/can’t handle, not to mention the diversion of your attention to issues and problem-solving.
It depends a lot on the physical and mental issues of the grandmother, and the level of responsibility of the son. Phone calls, managing doctor’s visits, grocery-shopping, hiring/supervising attendant, household maintenance, income tax, other accounting, tough emotional burden; or is it just a couple of phone calls and a visit per month?
Besides the weekends off (which saved my sanity), after our parents were both gone we went through the house and heirlooms ‘picking’ in turns in each room or stash, and at each stash my brothers said it was my turn to start picking. It was very low-key, but I knew why they were doing it, and the acknowledgement was good for my heart. And I got respect from the rest of the family for doing what I did, which I value.
So this is to suggest that there be some on-going support, payback, or reward for the cash or time he is putting in, and at the end a bigger slice of the pie (assuming there is one–you said money no object).
In fact it would be legitimate for him to be paid at full market rates, and this may ‘spend down’ the grandmother’s finances that will qualify her for gov’t assistance, thus saving the estate over-all the thousands per month that may otherwise have to be paid for nursing home care, if that is likely. And of course it would compensate the person who is doing the job.
If everyone has plenty of money, a serious symbolic gift might be fine, but (a) time and (b) compensation, preferably on-going or maybe upon the estate being settled, might be a very good idea.
If he resists compensation, remember that the burden slowly gets worse, and a person who cares for Grandmother and only wants to do right for her, can also begin to realize he is almost the only one doing anything, which can degrade the family’s feelings for each other. (I can’t tell how much this might apply here, you haven’t gone into great detail. But it’s best not to allow him to be noble about it.)