To think ex shouldn’t have left DS with friends for 3 days?

You say your DS knows them quite well, they weren't complete strangers and he has perhaps seen their house before...

It's not the worst thing

I'm not sure what the solution was, to be honest if you couldn't have him, his father couldn't have him and your MIL was away? Poor kid. He seems to be an emcumbrance to you all.

It might have been horrid for him - but I'd mostly be feeling awed that any childless couple would be prepared to care for a friend's 4 yo for four days. They deserve a medal.

Was your son really upset about it when he came home?

He knew the people and I can’t see why they are inappropriate, he needed help with childcare and he got it

It wasn’t ideal but in the absence of any other options, I think as a one-off, considering you have a 50/50 arrangement which presumably means he’s a hands on dad and someone you trust to make sensible choices, it’s not the end of the world. I wouldn’t go too overboard on having a go at him: you never know when you might need equal goodwill from him if you have to make a less than ideal childcare arrangement yourself.

Have you asked DS about it? You’re assuming he must have been uncomfortable, but a holiday with some nice people he knows who were probably pretty indulgent with him and will have gone out of their way to make him feel welcome and comfortable may have been something he took in his stride.

I do think you are unreasonable if they are people that their father knows and trusts, and your little boy also knows them and feels safe with them. But I understand feeling protective about your little one and in your circumstances may have similar feelings. It’s hard to be objective when it’s your child. What is it specifically that upsets you about this- is it that he may have felt homesick or is it something else?