I agree OP has shown nothing other than concern for her son, and right from the point of her original post her inclination has been to protect him.
But I can see why a PP should have posted this in a bid to shake her out of the nonchalance that's clearly coming across in her posts. It's likely that her horrendous DP is doing very real harm to her son's mental health, and the posts overall show a lack of conviction as to whether she's really in the right to insist her own child is welcomed into his own home and own room.
If anything else, hopefully the responses will have given OP that very necessary conviction. A bonus would be if they gave additional pause for thought. I hope it will also be made very clear to this domineering man that her son does not owe him 'respect'. He's not his father. His behaviour toward him has been anything but that of even a barely adequate parental figure.
He's quickly assumed a position of authority in a home that isn't his, OP, and now you claim to be afraid he'll refuse to leave? Men like this know how to choose their dupes. (I say that not to be in any way disparaging, I've been a dupe too in my own time). There is plenty in this relationship for him, he's throwing his weight around as if he's the householder, and he's very good at ensuring things are dictated to and changed to cater for his own best interests. Fuck the interests of the bereaved, mentally-struggling boy whose home he's coopted. And fuck, also, those of his desperately worried mum who just wants to do her best by her child, and he's trying to drive a wedge between them and inch him out of his family.
I've rarely read anything on MN that's disgusted me more. Take a look at the precedent this unempathetic, callous, emotional (and financial) leech has set over the past 4.5 years, then imagine what the next 5 might look like if he carries on in the same pattern.
Through my internet-stranger's eyes, it isn't pretty.