I'm new here, but have been combing the forums for advice for a while. It's a long one, but please help. 

I'm 30 this year, my partner D, is going to be 47. He has a son who just turned 11. Been together for 3 years, known his son 2.5. We have been through a lot in that short time, including a miscarriage around that start of year 2. We've been living together for 1.5 years. The house is tiny and there's not much space, but we got somewhere with a room for his son. I even decorated it with Star wars stuff for him as me and SS are both fans. 
Anyway...

SS has always been a problem. He is an odd child. I have a feeling he may be on the spectrum, as well as D. Not sure. He has very strange behaviours that can be irksome. This would be okay, if it weren't for the extra crap. Self centered, entitled, stuck in the know it all faze, clingy, rude, selfish, never listens, refuses to learn from mistakes or dicipline, and ontop of this... he scares me. Sometimes he will stand there and stare at me, the look in his eyes is creepy. He seems to totally lack empathy, but tries to fake it... poorly. 
BM didn't take kindly to discovering me. She punched D and screamed at him in front of SS saying some rather un-child friendly things. D left her because she was abusive verbally, and had become physically abusive, she changed after SS was born, and didn't want anything to do with D romantically or sexually. 
Without knowing 100% we believe she has told SS some horrible things about me too. SS comes out with awful things about me regularly, and just before x-mas he stole an octopus D's mom crocheted for me, and when D went to get it back found a horrible drawing of me made by SS. 
I have "ruined his life" and he doesn't want me to join in on any activities, his reasons always different and as stupid as "she wears makeup". He refuses to stay here, so his room has become a storage room. He has been violent towards D, throws toddler style tantrums, walks off if he doesn't get his own way then accuses D of putting me first. Anything I suggest is challenged, down to simple activities, like what TV show to watch. Whenever I say something factual, I am told that I am wrong. 

Over time this has become increasingly difficult to deal with. D has tried dicipline, of all styles, nothing works, he's had numerous converstaions with SS about me, but SS refuses to budge on his behaviour towards me. D and me are great together, we've really helped eachother heal, as I have mental health problems (due to trauma as a child/teenager, and an abusive relationship that almost killed me). I also have physicall illnesses, and struggle with daily life. When it's just us, things are wonderful. Recently D has been acting differently towards me around SS, and this just adds to my stress. For example he had a go at me when I said I didn't like the 13th Doctor Who infront of SS, because SS loves him. It's like... I'm not allowed an opinion that may upset SS.

So coronavirus. This is why I have finally shown my face here. I live in the UK. Today it was annoucned that schools would be closing until further notice next week. D has been working from home for 2days, and I don't have a job. BM however may still be working, so SS will have to come here during her work hours. At the moment, we have SS on wed' from 5:30-7pm, D has him at his mothers house (who is over 70 and will have to be careful now) on Thursdays until 8pm, and then Saturdays, times dependent on SS's whim. (Which I really don't agree with, and have asked for set times, but get told NO, by BM). I already dread these visits. It's only a few hours a week, but with SS's behaviour it has driven me to wanting to leave, or go hide. As I said, the house is small. All I have is my bedroom, where I've had to stash my PS4 as SS would mess with it. If he's going to be here more with this coronavirus, I'm terrified I will lose my shit. 
I don't want him here, I'm horrified by the idea. I know for sure, 100% that SS will not get on with work his school sends, will beg D to play video games with him. Will demand attention non-stop, and get in the way of Ds work. He is clingy as hell, and if I even open my mouth to say something to D, all you get is "DAD! DAD! DAD!". 
I am so worried. I actually WANT the UK to go on lockdown, so we can't have him here. 

I know people often say to women like me "leave him" or "disengage" or call us monsters, I can't stand this child and I am convinced if me and D break up, SS will be the sole cause. 
I don't care about these opinions. I want real advice, and reassurance. I am not letting a child ruin the first great relationship I have had since my abuser. Me and D are head over heels for eachother, and even with our 17 year age gap we are a perfect fit. Nobody will ruin that happiness for me, nor him. D deserves it too. He's a fantastic person, a great partner, and tries so hard to be a good dad. 

How do I stop myself going crazy? How do I get through this, when my source of relief (popping down to T K Maxx) isn't an option? How do I keep my relationship happy and healthy through this time?