Whenever I get what looks like an insurmountable problem, I look at it a different way.
Split all of the problems into separate sections and treat every one individually.
First of all, take away all the history of the situation, who did and said what previously doesn't make a jot of difference to the situation in front of you. Remove it all.
Second, look at the situation that's right now in front of your eyes, what is that exactly, describe it as best you can.
Third, what are all of the options available to you to resolve the situation?
How much of that can you do on your own right now? Implement that for a start and have a plan in place for any future stages.
Like everything else, you have to push for it to happen.
e.g. He doesn't want to go out.
You realise that's not good for his physical health or his mental health to be in his room all the time.
Up to now you've given him the choice of staying in 24/7.
.
How can you get him to spend some time outside every day? Well, initially not by giving him that choice.
Every one of the following things are pretty ordinary, but they will get him out of his room and into the fresh air.
Tell him, bluntly if needs be, that he's part of the family and he needs to contribute his time and effort in just the same way you and your husband do to keep your home running smoothly.
Do you have a garden?
If so, he needs to be put in charge of the family growing their own fruit and veg or a complete re-landscape project, making raised beds, laying a path, mending or painting the shed and fences, even weeding.
Find something that will occupy him outside physically and mentally for a couple of hours every day. It will build his stamina. He needs fresh air and especially VitaminD from sunshine. As his diet's poor, a multivit with mineral and extra VitD3 would be a good start.
While he's outside, teach him which birds visit if he doesn't know already, if he does, get him to observe their different habits in the garden. Teach him which plants grow in shade or sun, what they are, which ones are edible, which are medicinal, can he create and maintain a herb garden for you?
Can he keep a photographic record of the garden and the work he's doing, the plants and the wildlife in it, write a blog about it, even if it's not for public viewing.
If you don't have a garden, set him on doing exterior house maintenance, cleaning windows, any paintwork need sanding and re-painting? Pressure washing the drive, washing the cars.
Pegging the family's laundry on the line. Growing plants in containers.
Initially, you will have to set each task for him but hopefully he'll become engaged with it and begin to take an interest and suggest things himself.
In a few months, because of his new outdoors routine, he'll be stronger, fitter and healthier without having been pushed right out of his comfort zone.
It's a simple relatively cheap solution, do give it a try.