OP, I am so sorry, reading your pain is making me feel queasy.
It’s maybe too early to say, but do you think this marriage is salvageable (rhetorical, we don’t need to know). Regardless, it’s really raw so whatever you do next needs to be done with a cool head; that’s not easy if you’re under-fed and sleepless. It’s easier to take the next steps when you have a plan of where to go in place. You might find it would be easier to rest as well.
If I knew he wasn’t going to be my husband anymore I’d say the whys and wherefores of how OW behaved when confronted no longer matter. I’d take down the social media posting and take steps to protect my family. Possibly a bit paranoid but I’d be concerned about his immediate solution being for him to go home. Buggering off and leaving me with a devastated teen and a young child with additional needs thousands of miles from home is not a caring solution. Sadly, people sometimes go on the defensive when they’re caught out, particularly when they’ve told themselves ‘it was nothing/ it was over anyway’ and they realise the wronged spouse is not going to forgive and forget. 100% agree with all the PPs who’ve said you don’t want him back in the UK first. He doesn’t sound as apologetic as I’d expect given the atom bomb he’s set off in all your lives, and is already minimising and getting obstructive.
To give me thinking time, I’d hide his passport somewhere he’ll never find it, meaning he can’t unilaterally decide to go home. I’d hide the others too, so he couldn’t impede me or my kids’ return. I’d feign ignorance if he noticed.
I’d also look into my mum/ a trusted friend getting in and clearing my house of all joint financial paperwork, certificates, removing laptops, and having the locks changed.
If I could get that done, which is the most I’d do if I went home on the first available flight, I’d feel a little safer. I’m assuming this was a once-in-a-lifetime holiday for a little one, ie. Disneyland or something). If so, I’d try my best to rally (I know it’s easy to write, I know putting on a performance is going to need reserves you don’t have when you’re stunned and grieving). I’d take the little one out and give him the time of his life, and book a flight home for the three of us for a few days time.
It’s a shame for the kids to lose out on half a holiday but in truth the joy is already over for the older one, this is appalling for him, and possibly the most unforgivable part of the whole affair. Ultimately, a mother’s health and well-being is more important than a fortnights’ pretend fun. People in shock need space and adult support. The stress of this doesn’t seem sustainable alone in a different country. Imagine the man you married died, you wouldn’t stay, and if you were my sister or friend I’d urge you not to try and see the whole trip out in these circumstances either. Obviously, you know what you can cope with better than anyone, but please be careful of yourself.
I wouldn’t tell my husband any of what I’d done. I’d get my pal or parent to book the homeward journey on their card, I’d let him think the holiday is going on as planned with him taking younger DS out and about in the daytime and ‘you needing time to think.’ He’s not the person you believed him to be, and at the moment you don’t really know who he is or what he’ll do. IME most people feel stronger for regaining some actual control.
Best of luck to you and your boys, they are tremendously lucky to have you as their mum.