Like it or not, I think it massively changes the dynamic when it's a step-parent who moves in after you.
In a 'traditional' set-up, mum and dad would have got the house together (whether bought or rented, it doesn't make a difference) and established it as theirs, then the children would have come along later into that situation and, eventually, moved out and let the house revert back to their parents' ownership/control.
In this case, it was his and his dad's house, and now his dad has remarried, she has moved into THEIR house - so he has less privacy and less time with his dad, but no massive differences. Now, because of his dad's remarriage, that same newcomer woman has her eyes on his room in his home that he had long before she came along, for the new baby who has only come along because she came on the scene (of course, it was his dad's choice as well).
In a sense, he potentially has/had the worst of both worlds: growing up as an only child, which can be lonely and less fun (depending on the child and their personality, naturally); BUT one benefit of being an only is that you never need to worry about having to share and your room can always be yours whenever you come back to visit (or even need to move back in), unless your parent(s) move/downsize, of course.
Also, I know there's a widespread disdain on MN for anybody hoping to inherit when the time comes, but he may well see it that what would have been his inheritance will now instead go to his dad's (presumably significantly younger) new wife; then, she might subsequently decide to leave it all to her son and he gets nothing.