I had a similar situation here, as my stbx was planning on moving abroad to be with his fiance. Of course, this was his second fiance in less than two years since we've been separated. He'd already quit his job the year before and moved across the country to be with the OW, but a few months later she dumped him.
A few months ago I was told that he was moving abroad. He didn't bother to tell me when he moved across the country (his mum rang me and told me after he moved). He didn't bother to tell me that he was moving abroad, although a slightly amusing twist was that immigration there detained him and sent him back. He still hasn't mentioned it - he thinks I don't know.
His fiance from abroad is still visiting him regularly here (although he is very quiet about it - he is very careful to try to hide things). I'm just waiting for him to try it again. He hasn't paid maintenance in over 18 months anyway so I am not even going to deal with that part. It's just more aggravation for me, as he won't pay it.
MIL is vulnerable and he is living with her right now - doesn't pay rent or anything to her, she does all his cooking, cleaning, laundry. He intimidates her and she has memory problems and other health problems. The last thing I would ever do is put her in the middle, so I try not to discuss it with her unless she asks. I won't lie to her though, and she is horrified at how he is acting, but she is afraid to step in, as he is living with her and quite intimidating, and he is her only living child and she feels dependent on him (god knows why - he does nothing for her but create more work and stress).
It's tough. I guess with my stbx I have just had to accept that he is a miserable excuse for a father and try to minimise the damage to the dcs. He rarely sees them, even though he lives quite close. They are way down on his list of priorities. Doesn't stop him from telling everyone that I won't let him see the dcs, which is completely untrue. He just can't be bothered.
Get good legal advice, but also focus on yourself and the dcs. Don't let his fuckwit behaviour make you bitter or angry. I really struggle with that, but find when I can just ignore it and focus on my life, I am less stressed over it all. You can't change the way he behaves, you can only really minimise the negative impact it has on the dcs.