Am fuming for you. Like ICanSee says I really don't know how to begin commenting on what he's done .....
.... to present you with a fait accompli is just so disrespectful. You could say that about anything reasonably major, but when your husband disappears off for 3 whole weeks without any prior discussion or warning, AND your son wasn't even thought of, AND literally, in the weeks just gone, you have included his son in your hioliday arrangements ..... well, talk about bloody double standards.
Going anywhere for that length of time is going to take a fair bit of money. So that's the first consideration and unless you both have independent money and lots of it, I'm gobsmacked that he's taken it upon himself to spend so much without your input.
The second consideration is how he obviously thinks that he and his son are apparently more deserving of holidays than you and your son - because neither of you have been included in this particular jolly. Fair enough your DS declined the previous trip - am assuming he's an older teen so could stay at home alone - but when you've previously been badgered (by the sounds of it) to organise a holiday specifically "for" SS then the least your DH could bloody well do is consider your son in any plans for further trips.
I think he's treating you like a fool. He must have had the idea for this Europe trip before you went away yet was making out that his son was hard done by! How devious ....... so his son, and him, have now had 5 weeks holiday this summer and your son has had none. I also expect that had he been honest about ideas and expectations for holidays right up front, then quite possibly you might have booked a different sort of holiday for you all which your son would have wanted to go on - or, if your DH had suggested this 3 week Europe thing, you might instead have decided to go away alone with your son at the same time or something. But the way he's done this smacks to me of him taking any real choice away from you - there are all sorts of rammifications regarding how you might otherwise have spent family money, organised annual leave, invited SS's friend (or not), chosen a different sort of holiday and so on and so on. I mean, who the hell does he think he is ? ..... I'm not sure I've ever read anything quite so arrogant on here.
The way he's done this as well doesn't bode well for step-relations either IMO. I don't know how old your DS is but I wonder how he feels about what's gone on, whether he feels hard done by, left out etc ? I think your DH has acted like a spoilt brat TBH and suspect he's sprung this on you precisely so he could get what he wants, when he wants, without giving you any opportunity to put forward an opinion, discuss compromises, or be inclusive of everyone in one way or another.
Sure, it may be their "tradition" to usually spend a week together and that's fine in itself but courtesy and common decency dictates that he should still have discussed that with you because it could still have a knock on effect on other people and because you're his wife FFS with whom he shares a home, not some casual acquaintance! But 3 weeks is taking the absolute piss and I really don't know what to suggest you do next TBH other than have a long hard think about whether you can bear to remain with someone capable of being so utterly disrespectful, dishonest (this must have been in the pipeline when he was twisting your arm about the 1st holiday) and selfish. Not to mention how casually the holiday "needs" of your son have been so casually disregarded even though both boys live with you.
Probably trying to teach you to suck eggs if you're a counsellor but when he gets back I think you need to read him the riot act about what it and isn't acceptable. I don't mean laying down the law about when he can holiday with his son or for how long, but about honest communication between the pair of you. And about respect for you as his supposed equal, and respect for the joint household budget. And respect for the "needs" (wants?) of both kids. And about reaching joint decisions together when it comes to anything reasonably major to do with money, annual leave and the children. I'm sure you know all this obviously .... but if he baulks at that, do you really want to be with someone so selfish. The whole thing strikes me as a two-tier dynamic where he and his son are the most "important" people in the house and you and your son play second fiddle.
And that's before you even get on to the issue of house rules and boundaries not applying to both boys ....
Am so sorry, you must have a great deal to think about right now and I can only begin to imagine how let down and hurt you must feel.