Since the boy has shown that he cannot exercise the self control to stay off his phone at night.. he loses access to the phone at night.  The phone is turned off and you put it in your room.. where he can't get it.

He needs an alarm to wake up?  you can buy cheap alarms at walmart.  wind up or electric.

Having a cell phone at home is a privilege not a right.  He doesn't use it responsibly.. he loses free access to it.

I'd get separate cell phone plans. Let SO figure out how to pay for the overages on his own.

We have unlimited data because I don't trust OSS to not go over (or myself, for that matter). If that is an option, get it and have SO pay the difference. Then, SS staying up no longer impacts you and you can disengage from it. If he's late to school, then SO will finally have to deal with it.

You checking the phone and yelling at SO about it just prolongs the issue. Remove yourself from the equation and let the chips fall where they may.

This is what I have done with DH in regards to internet and screen time. I don't think the boys should have endless time on the internet, but DH doesn't see the problem. So I checked out of that fight. All I say to DH is that they better be up and ready if we have things to do with no whining about being tired, and they don't male noise at night. So far, that has been obeyed.

Why is his phone not taken away? He's been told several times about the over use of data and he continues to abuse it. Its obvious he doesn't know how to follow instructions. He has to be punished and it has to follow through so he knows that you both are serious. He's taking this as a joke because there are no actions and consequences behind it.

If your SO wants to continue to enable this I'd wash my hands of it and disengage. Let your SS wake up late to school, be tired and your SO to pay the extra money for the data.

DH and I used to share a plan and we would run into data issues because SS would play games on DH phone when camping, errand running, on the drive back to BM, etc.  We finally separated our phone plans and DH went with a different provider.  Since taking over his own bill, there have been several times where he has forgotten to pay it when it was due and didn't know until they shut his phone off.  It's nice having one less thing to stress out about, and if SS uses all his data I am not in charge of the bill

You can turn off data to his phone completely. I know thats more of a consequence than any skid would ever get

I had to disengage from SD because she is an obnoixious twat and DH never disciplined her.  And I told DH exactly why.

Now every time her reprimands me for something stupid, I say, "Wow, you're really good at correcting ME.  Lol!"

Shuts him the eff up every time, instantly, and he does it far less often.

Not your child, not your problem walk blissfully away and leave DH to deal with it.

log into the account and throttle SS's data but you know he won't

You've gotten plenty of good advice on the phone issue. I'd just like to add that learning about Karpman's Drama Triangle might give you insight and help your approach to skid problems. Google it, please.

Other posters have mentioned Triangulation, and it's huge in steplife. Heck, I've successfully applied my knowledge of it in other areas of my life. It's aaalll about positioning yourself in the best possible way to achieve a particular goal.

When you blew up at your DH, you positioned yourself as the Persecutor, the bad guy and mean ole SM. You lost the battle right there. It totally feels good to air your frustration, but accomplishes nothing. Always try to position yourself sympathetically; you're trying hard to ensure skidly gets enough rest, you want him to do well in school, etc. Make your DH want to rescue you instead of the skid.

I'm not suggesting that you forego disengaging, just incorporate an understanding of the Triangle in your approach to stepping.