Has he downright asked his mother why she thinks it should be 50/50 when the other son has NOT been contributing to it 50/50?

There should be proof through the payments coming from your husband's account that his brother hasn't been contributing to half the mortage... as there wouldn't be any deposits from the brother into your husband's account. But if the brother has paid your husband cash in the past then it would be harder to prove.

I would suggest your husband go to a lawyer if you can afford it to get something drawn up... or at least to see if there's a way to work it out that the brother has to sign off to your husband each time he pays.

Find SOME way to keep it more accountable, so that when the day comes that the house needs to be sold... all his missed payments need to be paid out before the 50/50 split. The past payments may be hard to get back, but something can and should be done for all the future payments...