Poor lad. I feel for you both.
Do you think if you asked his dad to leave on his own, he would? He doesn't sound like an engaged father, and although you have no automatic legal rights, as a PP said your SS is old enough to make choices. Would him doing 50/50 or similar work, perhaps?
I have been in your position and it's so, so hard. My son's birth mum walked out when he was 3. I came on the scene when he was 5. That was 20 years ago. He still calls me mum and lives with me at the moment even though I split up with his dad when he was 11.
During the split my son was devastated and his dad did his best to alienate him from me 😒 but we maintained contact - I had him every weekend until his dad moved several hundred miles away, and then we had daily phone contact and he came to stay every school holiday.
At his age, if you both want, you can easily maintain a relationship. And I'd encourage you to do so because you seem to be one of the very few positive influences in this lad's life.
Please try not to be hurt when he lashes out - he is testing the boundaries and probably feels really adrift at the moment with not seeing his friends and teachers. Is he back now? Hopefully he might settle down a bit.
When you tell him you're splitting with his dad, he may well lash out dramatically and push you away - this is a defensive instinct to prevent him from being further hurt. Keep the communications open and keep reassuring him that although your relationship with his dad is over, you still love him very much and want him in your life.
Good luck, it's a hard and sometimes thankless task being a stepmum (or ex-stepmum) in these circumstances, but it is also rewarding. My son is now 25 and we are incredibly close. But he certainly put me through it at the age yours is now! I would recommend trying to get him a CAMHS referral, they were incredibly helpful when we were really struggling. I'd imagine there's quite a waiting list at the moment though.