Thanks for the advice everyone.
I phoned the landline and they are where he said they were. I spoke to DS first, he is fine. I told him that I miss him and I just needed a short break and that I can't wait to see him.
He passed the phone to P and he was a pig, accusing me of stupid things. He thinks I've got rid of him (I didn't get rid of him at all, he chose to leave) because I've had another man move in
and insisted that I was 'boozing' at 11am this morning. I was sitting in silence, not moving, but he could hear me opening bottles with metal lids 
I told him that I would like this to be the end of our relationship now and that I won't accept him coming back. He asked if I'd like DS to come back or if I've had enough of him as well [bangs head against wall]. He said his parents won't let him stay and he has no money to rent a flat, which is true, but then again I have no money to pay my rent or bills or to buy food either so it's shit for both of us. I told him that he was just going to have to come up with something as us continuing to live how we have been lately is unacceptable and I can't stand it anymore.
He thinks I am being unfair and said that he IS coming back. I've told him he isn't and I bloody mean it. I've phoned Housing Benefit, DWP and Tax Credits this afternoon and told them I'm now a single parent. He has since phoned and now his dad is agreeing to a 2 week stay and then he will 'reassess the situation'
. Oh and he won't give me any child support for DS, which I knew anyway. Right now he can shove his money, I don't want it.
There is one problem though. He has a nurse visiting here soon to show him how to inject his new medication. The meds are in the fridge and they cost £400 each (2 of them). I don't know what to do about this. I doubt if she can travel to his parents house so I think I'll have to let him have her here.
SGB, you're right I should have left it until DS is back here. I didn't read your post until after I spoke to him. I really don't think he will pull any stunts as he finds DS very hard work and is probably struggling with him already. DS will definitely want to come back here with me. I don't know what a solicitor can advise me with at the moment so I'm going to hold off on that for now.
I would really love a hug right now. After last night's euphoria I am so tired and worried about it all. I miss DS like mad and I can't wait to see him on Saturday.
Separated, thank you sweetheart 
Annie, this is very typical behaviour from P. He has never been able to grasp that he isn't always the victim in any given situation. He can be very nice when he has his own way and I think this is why people generally think he's wonderful and I have no support. People see a kind, soft-hearted man who's P has stopped loving him - they don't see the emotional bullying and learned helplessness. From way back in our relationship I've felt like I am personally responsible for his happiness. I have had to put his needs first about everything because he is 'softer' than me. I am very tired of it.