It has suddenly occurred to me why he is refusing to go - he needs the money!
Unlike Sunak, Javid, Zahawi, even Rees-Mogg, he didn't make a pile before he came into Parliament and with a wife and heavens knows how many children to support. He desperately needs every penny of the PM's salary. He also doesn't have any home to go to.
So if he goes, bang goes the money, he has got to support his many obligations out of an ordinary members of parliamets salary.
That or is waiting for one/some of the wealthy members to club together to offer him a sum of money so large that he cannot refuse it.