So sorry OP.
I'd take this very much one day at a time, but be careful to remain in control.
- Firstly, while he is still in hospital and fully incapacitated, get control of everything financial and house related that you can, get into Ipads and the like, copy anything you think you'll need and get those messages backed up somewhere.
Once he is with his parents, it's likely that they'll shift opinion fast when he's at theirs, weak and sick and telling them a whole bunch of lies about the situation. And once he 'comes to' a bit, he's going to be thinking about himself and will quite possibly enlist them if he can't do things himself. Make sure that his parents can't access the house, for example 'Oh would you pop over and pick up my ipad/deposit box whatever - please go when Lost isn't there, it will only cause upset but they are my things'. Etc. Freeze joint accounts now, or, more usefully, move all the cash out so that you can access it but he can't for now. Try and get banking passwords if you don't already have them, or try signing in on his devices if he has passwords saved - and then screenshot details of his accounts. Find pension stuff, etc. Photocopy it all before he gets to asking for it.
Start telling friends and family before he gets there with the sob story.
Make SURE that anything he might be able to change passwords for etc - you get there first and change them.
Then you can stop and think. Just do the essentials while he is unable to get a jump on you.
I agree with remaining NOK, and try to remain on good terms with his parents if you can - it will be useful, and help the kids. I'd be blunt with them, if you can find the right moment and words. 'Look, I know that once X is living with you and convalescing that his wellbeing will be your main priority, I also know that the care is going to be a stressful time for you. I expect you to put him first, and it would be great if in return you could recognise that obviously I am not only extremely hurt but have now been left in a similarly stressful situation, namely caring for the children and their emotions and coming to terms with what he has done. I hope we'll remain on good terms even though there might be awkward moments ahead; my first priority is making sure the children are ok, so I hope you can support me in that'.