Piper, he's a bad 'un. I would laugh at his silly suggestion of a test and tell him to move on with his life.
His suggestion for the test is to see if he can get you to waiver.
I think you mentioned you own your house. This suggests that you may pay the bills too. Is this correct? I'm thinking he may not let his meal ticket go to easily.
Expect crying, suicide threats, rubbing your face in his free and easy get pissed lifestyle, anger, Mr nice, Mr nasty, insults and jibes. Watch out for the 'bad mum' comment, you know he feels he is losing when the words 'bad mum' fall from his mouth. He may try and make you jealous too.
Create a tick list and see if he manages all of the above.
On a serious note. Do the freedom programme and read 'why does he do that.' There is no point writing him letters, he will use your feelings as a stick to beat you with.
Take care of you and your children. He doesn't deserve any of you.