Depends if he’s doing it from joint money or personal money. We both have an amount of money every month that’s just our own to do with as we please.
We are also able to make individual decisions about joint money without consulting one another, as long as it’s not a big purchase- which for us would be £200+ as a one off or £50 a month recurring. It depends what you’d consider a big purchase really. Even then it’s less about the money and more about the fact it’s something we’d both likely be using, so we’d want to get something we both liked/suited us.
This wouldn’t be the hill I’d choose to die on. If he’d spent it on himself, maybe I’d raise it. Spending it on his child, even a grown up one, definitely not. As you’ve said yourself, you wouldn’t always consult him about purchases for your children, so that’s the established standard.
I would however have a good look at what my reaction to this was telling me and take action accordingly.
So if it was telling me that I wanted as a family, to prioritise saving more towards house purchase, then I would have that conversation. If DH was in a similar place, then that would mean reconsidering priorities and spending across the board, maybe setting targets and deciding sacrifices. I would have that conversation from a positive place though e.g. “I’m just realising how much buying a house together means for me and for our future. Can we look at how we can make that a even bigger priority for us?”. I wouldn’t link it to the phone and I would be open to the fact he might have different ideas, either about how important it was or what changes should happen.
Equally, if this is about transparency, or setting some limit to how much you spend without consulting one another, then either a positive conversation or just start being the change you want to see. Again, don’t link it to the phone.
If, honestly, it’s anything about the fact it’s a step-child or a grown up child rather than a young one, I’d be taking a look at myself. I mention the latter because your children together will always be younger than your step-child so it could be a covert way of favouring your own children without seeming to, even to yourself. And I’d realise it’d be best to explore that within myself or with a trusted friend with a view to getting to a better place about it, being fairer and more even handed in my outlook.