@VivienHarmon
I am on the fence here. On the one hand, I do know a few people with bone idle, directionless 16 to 21 y.o's, still living at home, who just can't be arsed and have no motivation to do anything. And they always cite stress and anxiety as an excuse to do anything (a few I know are even older - like mid 20's!) Doesn't stop them socialising with their mates, getting pissed every weekend, going on games consoles 5 hours a day, and watching netflix marathons. Most of them don't pay any rent or contribute ANYthing to the household.
Having 2 DC in their mid 20's, who have both been to uni, and got their degree, and now have partners who they live with (and have never lived at home since,) I can only IMAGINE what a PITA that must be.
I also agree that it's not fair to expect a child to just be a fully fledged adult at the age of 18, and make their own way in the world. It is hard, especially if you don't have a partner. (Having a partner halves living costs for a start!)
On the other hand OP, you REALLY sound like you don't like your stepson, and you don't want him there. AT ALL. Even if he got a job, you still wouldn't want him there. Reading between the lines, your posts are full of resentment for him. It doesn't sound like you 'love' him at all. Why pretend you do?
I guess this is the risk you take, starting a relationship with a man who has children. There is always a chance that they may want to come live with you. As a pp said, why shouldn't your partner shoulder some of the responsibility for HIS SON? The lad's mother has done HER fair share.
The lad living with you is a cross you will both have to bear. No reason at all for him to NOT live with you. I just hope your relationship survives it. Many don't. Many women who get with men with children, assume that those children will always live with their mother. Not always so. It is a massive culture shock when your partner's children come to live with you, and as a pp said, your partner will almost always put their children first. Before you, and before YOUR children.
Good luck. I don't envy you.