Brilliant advice above from Pinkdelight3
I would add to it. You do indeed need to tell him that THIS job needs to be turned down, but you then need to watch and learn VERY carefully from his reaction. Your permanent happiness may depend on it very closely.
If he's willing to listen, sees your point of view, and agrees (or at least recognises that you have every right to decline and that means that the job is off) - then good. Your interpretation of his motives as generally innocent and positive may be fair.
If you get sulks, maniplation, persuasion - the sudden appearance of additional factors or terms and conditions which mean that turning this down will be fatal - basically if he tries to railroad once you have said a fair and considered no - then you need to think VERY carefully about moving you and your children to be in his home country.
You call him impulsive, keen, just missing his kids, didn't think he'd get the job so didn't want to 'worry' you - all very, very telling phrases which make it clear as day that the dynamic here is (at least a little bit) - his MO is 'do it and ask permission later' and you make excuses for that.
The red flags that people are seeing here is that his behaviour isn't actually as innocent as you'd like very much for it to be - and that he gets his way by being manipulative. That's definitely partly the case, by the way - him not even telling you about the application then steamrollering it very quickly into a 'we will move next month' is an absolute forest of red flags and there's no other way to interpret it. No way on earth would I be ok with the way he went about that. It tells you a LOT about how he sees your relationship and how much he feels entitled to have things his way.
Once you move, you have put yourself entirely in his power - whether married or not. It's fine to say you'd intended to go anyway but there's more to consider. If you split, if you end up hating it, if he changes once he's there with all his family and you're alone in that sense- if he's unfaithful - if the dynamic with him being closer to his family and your inlaws being more in the picture doesn't work well - once you are there, the scales are weighted towards him permanently, as he will know and you will know that leaving will become a million times easier for him and a million times harder for you.
I will just say that I would very much not like to have my marriage suddenly put into that status quo with the kind of husband who doesn't tell me he's applying for a job in another country so as not to 'worry' me...
I wouldn't trust this man as far as I could throw him after this stunt.
I suggest you explain to him why this job will not be suitable - then sit back, watch his reaction, and judge from that whether you should trust him either.