DS1 is also academically able, but it is little use if he can not function in everyday life.
Have you spoken to sixth form about how they can help? Teens get 3 years of post 16 funding, so DS could re do a year or have a year out. (Technically he should be in education or training, but realistically no one is going to chase you.) Either way he doesn't have to sit at home doing nothing. That would be detrimental to his MH. As his MH allows he could focus on life skills, hobbies, volunteer, work experience, get a little job, enrol on a short course. With an EHCP funding lasts until 25.
If you get Child Benefit one thing to consider when deciding what to do in September is it would stop if he isn't in education.
Personally, I would keep him in education and apply for an EHCNA. If he isn't well enough to attend then he isn't, but don't take him off their roll. If he becomes NEET you may struggle to get him engaged again.
I would try to get him out of bed during the day. Sleeping during the day and awake at night won't help his MH. He needs good quality sleep, going to bed at the same time each night and getting up at the same time even if he hasn't slept well, otherwise you get into a vicious circle. Laying in bed for so long means he'll be losing muscle. Muscle wasting is something you REALLY want to avoid. It is 10 times harder to rebuild muscle and simple tasks like getting dressed or in the shower will make him tired.
Being in his room 24/7 isn't good for his health - physical or mental. Try to get him out of the house everyday, even if it's just the garden. This will be difficult to begin with. Once someone with MH problems gets out of the routine of doing something it is harder to get them to restart, but with perseverance it is possible. You can do this little by little. "DS I've hurt my foot, will you put the bin out please?" "DS the logs are too heavy come and carry them please." "DS I need your help to get everything done before dinner. The fence needs painting and the patio jet washing. I'll do one you do the other, which would you prefer?" Get everyone to help declutter the garage. This type of thing will get him out of the house for fresh air and Vit D, and get his body and mind active. You should get your other DC to help in and around the house too if they don't already so DS doesn't think you are targeting him.
Unless he's underweight forget about food for now. If he'll have it give him a multivitamin and extra Vit D. If he's underweight he needs a referral to a specialist dietician.
Can DS talk to you via text if he finds it easier?
There will always be someone to listen on here. MN's SN boards are also very good.
DS1 had an awful night which stemmed from me getting yesterday so wrong. I could kick myself because I really should have realised.