OP - almost everyone is on your side and is genuinely concerned about the welfare of your son. I have read all your posts. Every single one and most of the replies from people too
The reason people are still commenting is that you don't seem to understand that the things you have said are demonstrating HUGE red flags about your OP and it is raising concerns because from what you say, it may very well be that your son's Mental Health is at least in a large part, the result of you living with your DP:
I will tell him to leave whilst DS is here, but I'm not sure what to do if he refuses
I hope it doesn't come to that though as DS has said he wants to go home not to a hotel! ( in reference to You and your DS staying in a hotel instead of your DP and his children)
And that is in addition to what you say about your DP absolutely not understanding and not caring about your son's needs right now
It is YOUR home, not your DP's and to be worried that he'll refuse to leave, or that YOU and your son will have to move to a hotel instead of him and his daughters is indicative of how much of a hold he has on you, now he thinks he has his feet well under your table. This does NOT sound like a happy, healthy, normal relationship and will NOT be helping your son's state of mind at all.
You may not like acknowledging that, even to yourself. It's a hard thing to do, but it's what the vast majority of people on here ( almost 100%) are thinking and saying, based on all the things you have said.
Yes - you are obviously very concerned about your son, but it sounds to us 'bystanders' that you are trying to compromise to help your son AND keep your DP happy. If you really want to make your son your number 1 priority then you need to listen to what people are saying - which at the very minimum is that DP needs to at least not be living in your house, while your son gets the chance to deal with his mental health issues - and that may take weeks or months. Not a weekend or a week - That isn't going to solve anything. For your son's sake I wish you could see that.
I feel for you, I really do, but I don't understand why you can't see that? He's NOT your No1 Priority while your DP is still living in the house. I'm really sorry, but the saying 'There are none so blind as those that will not see' springs to mind. And I fear for your son in this situation, I really do