No judgment from me. I've also had a DS who was like this for a few years and I know how incredibly difficult it is to get them to change.
It's not realistic to expect him to move out and be independent at the moment, so you'll just have to take baby-steps with him. The main thing I think is to shower him with love and your attention. You need to (gradually) make him feel that he's an OK person, that he has skills, that you enjoy his company etc. So that means really, really bigging up any little positive thing he does and not pointing out his failures (because he will be aware of those himself anyway.)
I started by having a rule that DS could not stay in his room all day - he had to eat at the table with us for dinner as a bare minimum. We would make sure we had happy, interesting conversations and I would ask DS for his views and involve him in any decision-making when the meal was happening. The next step was getting DS to be responsible for making one evening meal for us every week. I had to make this easy for him by having a selection of very easily-prepared things in the fridge that he could choose from. I would make a big fuss about how delicious the food was and how much I appreciated him being helpful (even though the food was often hideous!) The next step was getting him to go to the supermarket to buy the food he wanted to cook, once a week. Later, I gave him a budget and asked him to choose enough food for 2 or 3 evening meals a week. All of this was to build up his self-confidence.
I would suggest that you schedule some time to go out with your son, just you and him. Is there somewhere he used to like to go or an activity he enjoyed? Get your DH to look after your other son, and just spend time regularly, one-on-one with your older son. Don't make any negative comments about him or anything which might stress him - just focus on enjoying each others' company. Can you go on a short holiday together or even a day trip out, so he gets a change of scene? He needs to sub-consciously realise that there is a whole exciting world out there, outside his bedroom.
When DS was feeling a bit better, I then told him he had to choose an activity which was out of the home to do once a week. Look online for groups, sports, meetups: anything that might catch his fancy. If he's a gamer, would he like board games - there are several board games groups in cafes/pubs these days? DS decided he wanted to learn a sport and I went along and learnt it with him (even though I wasn't interested at all!!)
His behaviour sounds like depression. Even though you say he won't go to a doctor, you could phone his GP and express your concern. Our GP phoned my son and did an online consultation. He's now been on anti-depressants for years and they make a difference to his mood.
Hope some of this might be of help.