Is the mortgage note in both of your names? How about the deed?
Mortgage is in his name only. My name is included on the deed. That was actually a result of 2005. My name wasn't on the deed at the time. He tried to kick my DDs and me - PREGNANT - out of the house back then. One condition of R was that my name would be put on the deed.
Then I would inform him, in writing, "Just so you know, @ $1300 I will be unable to continue to make the mortgage payment. I guess we'll let the Ls sort all that out, but I thought you should know since the note is in your name."
You could even itemize what you can reasonably cover with the $1,300, in terms of shelter, food, and other family necessities.