to think my mil is unreasonable for not giving ExH address

[Fantasyland]

Will try and keep story short.
Exh left nearly 4 years ago after another affair, he has been an utter t**t since then messing our son around with contact and letting him down.

He has moved further away recently and given up his job to move with his girlfriend (o/w) who is setting up a business and he claimed originally he couldn't pay any more maintenance as unemployed but then after using more excuses to our son that he was busy setting up her business he has admitted he is working for his girlfriend (who he still claims is his friend!) part time and he is paying me £11 a week.

He is always late paying and i dont trust him to be honest with what he is earning and pay the correct amount as after nearly 4 years he is still claiming that its his friend now, they were together but they split up and live as friends and he is employed by her , not that he is a joint business owner.

I want to go through the csa so they calculate a fair amount but he wont tell me his address and his mother wont tell me the address even though she knows he has been an utter arse and left me financially in the shit by now only paying £11 a week because he chose to move and give up his job.

She still sees our son but exh rarely does, aibu to think she should give the address for csa?
I also predicted he would do this when he moved and see him stopping paying soon as i have no way of tracking him down

Parents will always prefer the interests of their children over the interests of a DIL/SIL - and it's natural that they should. YABU to expect her to do otherwise.

There are ways round it, I hope they work, but if they don't, they don't, and you might like to think what you would do a generation from now if you were in her shoes.

The woman should be ashamed she has brought up such a scumbag who has affairs, dumps his kid and tries to get away with paying virtually nothing for her grandchild.

YABU, she seems to be condoning him relinquishing his responsibilities. If any of my DSs turned out like that, I would fully support a DIL and make sure she has the address.

In normal circumstances the mil should not give out her son's address but this is not normal. The mil's priority should be her grandson. I would not give my son's address to anyone but if he was avoiding paying for his son I certainly would give it out to an agency who wanted to contact him about it.
The mil could do that if she won't give it to OP.

I don't think I would want a son who did not care about his own child.