Your ex is unreliable & it isn't fair to any of you.
Glad you're meeting with a solicitor to settle things.
That being said, it isn't fair for your current partner to put you in the middle.
He sounds selfish, immature, has anger issues bordering on abuse.
Your kids don't need to be around this either, not a good male model.
Is this someone you're really sure about having a long term relationship with?
Sorry to be blunt, it's the kids this is harming the most.
Wish you the best.