My ex is an abuser, I actually would not put this kind of crap past him.
abusers vary only slightly in tactics, depending on the chinks in our armour using only what works best against us to make us do what they want us to do.
He knows that seeing you will be uncomfortable, he knows that HE wants something that you won't want to do for him. Or if he is ill, he wants direct sympathy and attention. he wants to see you nervous, he wants to feel the power of summoning you and you trotting along to hear what he has to say... it's a power trip, I will stake my sausage sandwich on it (it's Friday, it's my weekly treat, I've really earned it, its the most precious thing in my possession atm) He needs to feel important. He wants and needs to be in someone's thoughts.
My DS dad only calls DS when HE needs some kind of there-there. It's never about DS - he doesn't even call on DS birthday, not unless it means HE gets the attention he needs - he's had a cold or whatever.
Trust me, you will get yourself all geared up to go there, you'll get there and he'll trot out some pile of crap and you will wonder wtf you wasted your time and energy over.
I have the inkling he's potentially demonstrating how he can still put you (and now your DH) 'in your place'
email him back, cancel the plans, no offer to reinstate them. If he says he has something to say, he can email it, otherwise, you really don't need to engage with him at all.
Even if he'd been dropped on his head, struck by lightning and had a visit from Marley's Ghost and was desperate to genuinely apologise and atone for his treatment of you, abandonment of the boys etc he would know and accept that he has absolutely no right or expectation that you will hear him.
My guess is that HE feels bad on some level - perhaps because the new wife/baby are making him look shit because he walked away from your DC, and probably knows that they are better off without him. Someone has said someting to him about his other kids or something has reminded him...
Or maybe he is bitter and resentful the fact that someone is doing a better job than him, and he wants to claim some kind of credit somehow.
It will be about him. You can be sure of this.
You owe him nothing in the way of courtesy or politeness, genuinely.