Kali has made the point I was coming on here to make, chocoholic.
Your dh has lost his mother this year, as well as your FIL losing his wife. I'd guess that your dh really wants to be with his father over New Year's Eve - he needs that. And he doesn't want his father to be waking up alone, on the first day of the New Year - the beginning of the first year in which his wife will never have lived.
They are the two people who loved your late MIL the most - would you really deny them the opportunity to spend NYE together, and for your dh to know that his father isn't alone, just because you are worried about not getting enough sleep, and being tired the next day?
Have a nap once he's gone home. Or ask your dh to get up with his dad whilst you have a lie-in. Or put your own needs aside and look after an elderly bereaved man, and a man who has lost his mother, and who both will need a bit of extra tlc at the end of the year in which she died and the start of a new year in which she will never have lived.
You might also want to consider the fact that, if he's stayed up late, to see the New Year in, your FIL might well sleep later than he normally does.