It's a difficult one as you have clearly tried to take the high ground and let things pass for the sake of your son, where going by the facts you have presented, you would have been well within your rights to kick up a hell of a fuss.
And re this weekend, it probably comes down to your son, again. Has he "suffered" (wrong word perhaps) or is he feeling sad, by not seeing his mum for the past week (or however long)? If so then biting one more bullet for his sake might be the right course in this instance, but you should agree some extra time with him in return. Alternatively stability and routine might be what he needs most, in which case you stick to the agreed schedule. It's difficult to comment from the outside.
Either way, you do need to have a conversation, sooner rather than later, to set the ground rules. It is absolutely wrong, where you have joint or shared custody, for one parent to take a child out of the country without the other parent knowing, whatever the circumstances. Full stop. No excuses, no exceptions. Obviously if you can have this conversation and come to resolution through mutual agreement then so much the better. But if you need to take it legal you have that option (at least to clarify your rights if nothing else), but try one-to-one first.
What is your relationship like with your ex's mother? Or any other close relative of your ex? Just trying to think whether there are any intermediaries you can rely on to help support things from your perspective?
It sounds like your ex knows she has messed up but is happy to avoid having to discuss it. This is where you taking the nicely, nicely approach vs challenging it head-on becomes a tightrope balancing act. Good luck.
