I am not a parent in this situation, but my husband (now nearly thirty) was very similar to how you describe your son when he was a young adult. He had no social life, dropped out of college while studying twice due to mental health issues. He worked various retail jobs for a few months at a time and contributed financially and practically to the family, including lots of driving and shopping or elderly grandparents. He was generally a bit aimless, and didn't have any concrete goals. When he was 22 or so he moved to a city with me after years of knowing each other online and visiting. After a year of adjusting and working similar types of jobs as he had been before, he harnessed a long-term hobby/interest and applied for a job in a related field. As he had no qualifications but demonstrated skill and knowledge, he was hired in an entry-level position that allowed him to demonstrate his value, and he was promoted multiple times over a few years to the job he now holds, that he enjoys and finds satisfying, and also has him earning in the top 25% of income earners, which allows us to live comfortably as a single-income household due partly to my disability and partly to lifestyle preference.
During those years we got married when he was 24, and enjoyed our time living in a big city (though he still struggled socially and exclusively socialised with work colleagues). When he was 27 we had saved enough to purchase our first house, and left the city for a small town closer to his family. We purchased a three-bedroom house with a driveway and a garden, which had felt like a pipe dream when we were first living together and claiming universal credit to top up his minimum-wage income. He works from home and is very happy, and we've made friends with two other couples in our town, which would have also seemed unbelievable to us five years ago! We are now trying to conceive our first child, and my husband is looking forward to fatherhood.
His parents have been incredibly supportive and proud of his acheivements, especially as they were worried that living away from home and even working more than a part-time job would be too much for him. We spent a lot of time staying with them when we lived in a city, and having the option to be absorbed back into his family's day-to-day lived for a long weekend certainly helped him to decompress during stressful periods at work. However, his parents very rarely pushed or encouraged him to do aything - it's one of the few things I could criticise them for. He feels that things might have been different if I hadn't encouraged him to step out of his comfort zone, and I tend to agree. My husband's sister, who is a year older, has suffered from similar mental health problems to him, as well as a lack of direction and clear goals. She still lives with her parents, with her primary-school aged child, and has not worked in more than a year. Her parents fully support her, and don't push her to do anything for fear it will worsen her mental health.This is bad for everyone involved in my opinion, and has severely stunted her growth and has put tremendous financial pressure on my in-laws.
Excuse my long reply, as I wanted to give you a fleshed-out example of how these sorts of situations can end up. Your son has time ahead of him to discover how he wants his life to look into his twenties and beyond, and time to make it happen. There might be many false starts, but with the right support, he will thrive, especially as he's already shown the positive traits you outline.