That he can't keep doing this ...

[highheelsandheadheldhigh]

DH and DSS (young teen) have hardly seen each other in weeks. (Another story) DSS arrived Friday, seemed great, happy, settled. Went to a family party Saturday, DH and DSS were messing about in the pool. And DSS displayed careless behaviour. DH told him off, not OTT or in front of everyone. Next minute he's out the pool and on the phone to his mum to pick him up. Which she did. AIBU to think he can't just go running home the minute things don't go his way? How do we manage this, it's always happening.

Massively depends on why they haven't seen each other

Mother has prevented him seeing him

I think it's really hard when the child isn't living with you because it can put more pressure on all of you to really enjoy the time you have together.

Do you think you could get DSS Mum on side?Not always easy I know but it's very easy for a relationship to be lost by the non resident parent,my 2 oldest DS's 22 and 20 never see they're Father anymore because he didn't bother enough whilst they were growing up and now it's to late and he regrets it and they call my DH Dad because he's always been there for them.

If he was playing up and all your DH did was to ask him/tell him to stop then he has overreacting but it could be a build up of not having seen each other for a while and just general silliness from a child.

I would try and enlist his mother as an ally.. and hopefully she will support the contact and won't respond by coming to get him every time he does this. After all, she probably looks forward to some time to herself...
If she's not cooperative, (and maybe that's the other story) you and your DH need to talk to DSS about it, presenting a united front, along the lines of your house, your rules. And be frank about him spoiling the visits by this behaviour. Point out that you don't suppose he rings his mother up every time he gets told off at school. So he will conform in certain contexts.
It doesn't seem reasonable to take his phone... (but I would be tempted) but you might be able to strike a deal about when he rings his mother. Teens are all about deals and negotiations.
Ultimately though, you might not be able to change this in the short term, but in the interest of maintaining the relationship with his father, go along with it for now. Chances are, he will grow out of it. And I agree totally with ohtheholidays comment about loss of relationship with birth father being all too frequent; it happened with my DS, too.

Why has she prevented DSS from seeing you? Is it because you are OW? Difficult for DS in new house if his dad left for OW. You are unlikely to get massive support from the mum to make a new parallel family.

Not the other woman.
Can't get his mum on side as she sees it as a victory every time she gets called to the rescue. We tried saying you have to stay etc. but he cried like the world was ending for days completely ruining everything anyway!

Honestly op- I don't think there's a solution without the mother being on side. If she's willing to pick him up when he calls- bar physically stopping him- there's nothing you can do.

Which is super shit. I'm a stepmother too- DHs ex would NEVER come and pick up step son if he called her crying after being told off, and he's 8. Same as DH would never do the same when at his mums.
But! This is only because they co parent effectively with the needs of their son first and foremost. United front and all that.

If the mother is point scoring or willing to facilitate this then it's a shit situation all round. Sorry- that's not really advice is it