By the time he was 5, she was in a good position with a millionaire partner and a step sibling. If his father had been interested in investigating residence, I think he would have been doing that when she was in bedsits and selling toys for food, as there would be no way a court would order such a massive change when he was 5/6. After that, she had money, visibility, another rich partner and he's not known anything other than visiting/staying, hence the 50/50. I'm sure he isn't in danger, I'm sure he's fed and loved. Perhaps, as kids do, as he gets to 14 or so, he'll do the 'I'm moving in with my Dad' thing, but kids do that for such offences as being told to tidy their rooms, never mind because their mum is in the media. And she doesn't hawk his face out for sponsorship, so he has some level of relative anonymity, especially if he doesn't have her surname, when he moves on to secondary. Chances are that if he is bothered, he'll just refer to his stepmum and not mention her in school.
There isn't - unless she actually SHs when he's alone in the house with her or has a real breakdown necessitating hospitalisation - any realistic grounds for making a change in residence. And even if he did express a wish to move and did so, because she hasn't put him in every photo and live, it would be simple to keep it private (as it should be).
She's not a parenting guru, she's not a Mummy blogger, so I think the standard of care he has (because I'm sure it is perfectly fine) should be left out of slagging off her recipes, smegs and sideboards.