Find out what his goals are, and then outline the steps needed to reach his goals. Put deadlines on the steps, but make sure they are flexible deadlines, because stuff happens. Periodically review the goals and the progress made toward reaching them. Adjust the goals and the steps, if necessary. The ultimate goal (for his sake as well as yours) should be independence.

My son came home after graduating from college. It ended up being way too many years before he got a full-time job and moved out, and I think we are all regretting it now.

To the OP

Fellow mother of a Bi polar 21 yr old .. We still have our struggles but after an early on
set of Bipolar at the age of 10, I am better at being supportive .

I hope things go well , and the ups and downs are less.

The poster above has a great suggestion about identifying goals. He may need to start out slow. How about a volunteer position as he gets settled. That can only help - when he is ready to move forward he will have an answer about what he has been doing with himself (and he will have additional skills .

Some can never work unless from home or very close.
I was one.
Then some can be president and muck everything up...
Play it by ear.
Observe.
Don't push.

and demanding careeers.

I agree with your very thoughtful advice in your post, but the parent also has to avoid the overlay of pessimism. This is a very hard balance. I know, having been the parent in this heart-rending situation.

What occupies the mind seems to be one of the issues. The completion of a task and it's results appears to create better self worth and personal satisfaction. Listing with a goal in mind is a task that can, in itself, result in self satisfaction.

so do i.

I'll jump in, with no direct experience, but seems to me, encouraging finding 1 part-time job might help him in the jumping-in process. And I'm not sure how 'normal' full-time employment actually is, these days, for 21 year olds; seems like a difficult goal for MANY.

full time employment is not the norm for that age.

I was in college, but all I could get was Dunkin Donuts as a part time job simultaneously. The only reason I ever got into the technical field was because my step dad was able to put in a good word for me at a factory and that was at the age of 23.

Before my son had his episode, he was able to get full-time at a restaurant. It did take time, however. I won't push him past his limits. I love him and his stability is more important than a paycheck.