AIBU to ask DH to stop doing this and is there a nice way to say it?!

Agree
Why not ask SS if he wants to join you.
Your dh always doing the asking will make SS think you don’t want him there.
When you just want to go out with your dd just tell ss why.

I wouldn’t be leaving anyone out tbh. I’d always ask

Lol, exactly. As if. Let's imagine it's a football match. And then as if he would ask his friends to help him with the wording to politely point out to his wife he wanted 121 with his teenager.

Why are you pussy footing around trying to find a polite way when what he is doing is outrageously impolite?

Next time he’s taking DSS out, send your DD along too.

Yes there are - it's much kinder to offer the opportunity to enjoy some fresh air and spend some time with their sister though isn't it?

Yeah. Which is totally normal in normal families.

A thirteen year old is quite capable of entertaining themselves at home without parental input. Why is it unreasonable of OP to want to spend some time with her DD alone ?

He’s WORKING on his CONTACT TIME with HIS SON. He doesn’t ask OP nicely if she’ll do his parenting for him so he can work.

Just tell him. Some good suggestions already given. How often does he take them out separately or together? How much parenting does he do of either of them? Why do you feel so awkward being honest with him?

Indeed. Shame his own father wants to palm him off to his stepmom at every given opportunity and expects OP to step up every single time with no discussion or even a heads up.

In the example you have given you DH was WORKING at home. Not lazing around. In this situation i think you are being unreasonable not to invite your stepson and your daughters brother with you yes.

100% agree.
The old, old assumption that she's lumbered with one kid, so adding another one won't make a difference to her but will benefit him.

I’d just say straight away “Oo sorry no but I don’t think SS will enjoy this, we’re going to xyz and he’ll be bored!” And then glare at your husband. It nips it in the bud in the moment, and then later on when the two of you are alone, just say “I hope you don’t think I was being harsh on SS before, but you shouldn’t put me on the spot like that in front of him, as I felt bad saying he couldn’t come but it really wouldn’t have appealed to him, he would’ve got bored and ruined the day for D. Next time can we organise a family day out in advance?”

Why bother trying to find a polite way? Tell him to stop, the more impolitely the better.

This. In fact, having had this conversation, if he ever invites them along with you again, immediately counter it with a dad and lad idea. 'I think Billy will get bored at flower arranging but you two could go and see Captain America together, father and son time, eh?' His son genuinely does need quality time with his dad, and he can hardly say you're not allowed to issue invites like that if he's done it for you.

I've got 4 DCs and if DH is home i don't always take all 4 out.

It's no big deal just say when i go out with DD please don't ask DSS if he wants to come with out checking with me first. It's nice to take one kid at a time.

Say "DH, can you not keep telling DS to come out with us? It can be really difficult to find something both him and DD enjoy. He gets bored and it just doesn't work. If I'm doing something I think he will like, I'll ask him myself"

I suspect this is a problem that will fairly soon resolve itself as DSS heads through his teenage years and prefers the company of his friends.

I would, however, point out to DH that activities suitable for a 5 year old are usually of zero interest to a 13 year old, so it isn't a good mix and he needs to stop assuming it is OK.

Does he often take all the children out alone and leave you to have peace alone at home? And if not, have you pointed this out to him..?

Does ss live with you or is his dad trying to shoo him off during limited contact time?

I mean if the teen stays in the house that surely doesn't affect his dad's ability to work?

Perhaps he needs to organise his work better so he can spend time with his son?

She's taking a 5yr old out, they have different interests completely AND she has no parental responsibility for the step son, his contact time is for him to spend with his actual father who is the one palming him off.

Stop pussy footing around this lazy man, you’ve asked him repeatedly so it’s not that he doesn’t know. His child is there to see him but he’d rather you parent his child because he’s prioritising other things.

Absolutely this! ⬆️ I can't ever imagine taking one child out but not the other without at least asking. My parents did this when I was a kid, I've never forgotten it. When you married your husband, you must have known he comes as a package with his son, please don't either of you leave him out, it can be really damaging.

I agree with some of the above- get defensive right back.

’aren’t you wanting to spend time with DSS? Poor DSS, I’m sure he’d love some one to one time with you. We’re going to the play park so he can come if he likes but it won’t very fun for a teen- he would have a lot more fun with you if you organised something nice for him’

Indeed, poor kid, it's a shame so many men are so lazy with their own children and try to defer responsibility to the nearest woman.