OP, what is happening to your son sounds like what happened to me. I absolutely crashed, left school at 15. From my perspective I felt extremely unwell all the time (sick, dizzy, faint, in pain) and had funny "episodes" that I didn't understand, I had no idea what was happening.
My mum, like you, just left me sitting in my room with zero help. She didn't even have civid as an excuse. I didn't want to see doctors either, I was scared and any interaction caused more symptoms. I was housebound for 10 years just rotting.
Anyone remember that story about that girl who was just left rotting on a sofa at her parents house, and she died in that very place, but with no evidence of being confined or trapped? And everyone wondered how that could happen? I know exactly how that could have happened, as it was almost me. I barely moved. Movement caused pain, distress. Every waking moment was torture so I just stayed absolutely still. I also wanted to be euthanised.
When I was a child, what I needed was an advocate. Someone to get a doctor to me, or at least look at my case - and stick with me. You're past that now I think, he's too old - you can try though. Perhaps you should try now.
Don't expect him to take responsibility for himself. He's too unwell.
I eventually did get myself going, however, but I was 25 before that happened. It only happened because of online friends supporting me and helping me. It worked very, very slowly. Your son NEEDS someone on his side, to help.. Not push, help.
If you kicked him out, he would likely die on the street. If you take away his only contact to the outside world (phone) it'll only get worse - it won't motivate him. No motivation can get someone out of that, the body and brain are too fucked up. I know I would have died on the streer. I didn't care about myself anymore. As I said, every waking moment felt like mental and physical torture - I will point out, I wasn't depressed. I was in pain, mentally and physically. It was only talking to online friends which slowly worked to pull me out and try. Again, very slowly. No one else helped.
My story is long, I won't bore you with it, but I eventually (very late) got diagnosed with multiple mental health disorders and "chronic fatigue syndrome" - and now have an autism assessment, as they think all my "mental health issues" may actually be undiagnosed autism and not receiving the support I needed. It all fits, but I am still awaiting assessment.
If it helps, I now have a job (work from home is all I can do, but it's still work), house with a mortgage, and getting married in a few months. My life doesn't look like everyone elses, and I still struggle, but I have one.
There's a person inside your son, and he's in pain. You may have to be cruel to be kind - contact the doctors about him today, and stick with it. Fight for him. It'll take time.
And hey, if you don't, maybe in a few years he will surprise you. Everyone left me to rot, and I surprised everyone.