How do manage DS's sadness about DP not living here anymore?

[FruitString1]

My DP and I separated over a year ago, after 14 years together. We have one DS, aged 10 years. Basically DP was emotionally abusive, our rows were deeply affecting our DS, and life had become unbearable for all three of us. So after a couple of years of really agonising soul-searching, counselling and prevarication, I asked him to leave. Things are a lot better now. But my DS has recently started to lie awake crying every single night, begging me to let Daddy come and live with us again. Possibly because over Christmas DP spent quite a bit of time here (which all went reasonably OK, thankfully, but am glad it's over). I don't know what to say to my son, when he seems so so unhappy. There is no way I could ever let DP come back to live here. I have considered it, but I just can't do it. DP started a new relationship a few months after moving out and although that has now finished, it really moved things on to a new stage in life for me, and personally I would be happy to have no further contact with him. But we do have contact because of our son, who stays with DP once a week.

What makes it worse is that our son has high-functioning autism and at nights he is very prone to anxiety and distress. It's just awful and heartbreaking. I have tried to explain that Daddy definitely won't be coming back to live here, but it's very hard to explain why - without casting all the blame on DP and I don't want to do that. When all's said and done, DP is still DS's father and they love each other. Unfortunately DP has been telling our son that he really wants to come back home but that I won't let him.

If anyone else out there has experience of successfully explaining a similar situation to their child, especially a child with ASD, I would be so grateful for advice! Thank you.