It is very very hard and the OP has my sympathy.
Parents like the OP whom have tried everything over the years, are left with an adult in their home that sours everything, is an awful example to other siblings and a relentlessly negative presence in the house.
In my experience the situation will continue for as long as you put up with it.
OP, parents like you become hostages in your home.
They rule the house, contribute nothing and often simply will not more.
I had a couple of lovely neighbours with sons like yours. Never married, wouldn't move out and I was told by both lovely ladies it was so difficult.
They expected to be catered for every day and the women never had the peace of living on their own.
They both told me separately it was a huge cross they bore.
I used to see them walking up and down to the shops daily.
Both dead now, sons probably miss being skivvied after though by their late mothers.
OP, in your situation I would reach out to Adult services and 101 to see how you can get him out.
Tell him you want him out and are making active plans to have him removed.
Do it for your other children who need you well.
He is far too comfortable, why would he help himself?
Only parents whom have never dealt with a version of this can understand how soul destroying it is.
I do suspect a combination of supreme laziness, selfishness, depression and a personality disorder of some kind.
Having seen it up close and read so many threads on here from women that end up married with children to these men, who masked long enough to live together and have children, I would hate for my daughters to end up with a man like this.
Since Covid I believe there are huge numbers of young adults behaving like this.
Parents in effect carers for such children to the huge detriment of their own mental health.
However harsh it may read to some who have no experience of this, maintaining the status quo will not help them.
I think OP some decisions need to be made and he will unlikely make any attempt to be a part of any solution whilst his needs are being met by you.
Mind yourself. You really have my sympathy.