This is a common challenge for many young adults and a frequent topic in therapy. Making the transition from high school to work or college can be daunting, especially for those who function best with a structured schedule and found comfort in the routine of school.
To start, it is important to figure out the reason he is isolating. What is behind his behavior? Does he have difficulty initiating? Is he anxious? Is he depressed? Is he too comfortable and dislikes change? Once you figure out the cause you may act accordingly. It may be that he needs to see a therapist or medical doctor to address the underlying problem.
What is your son doing in his room? Frequently room time is spent online or playing video games. It is very likely that your son may be feeling anxious and overwhelmed, and is not sure how to move forward without the help of an outsider. Often young adults have problems with initiation as they fear failure, have had previous negative experiences, or don't know where to begin. Staying in the house provides comfort and requires little change.
After you learn the cause of the behavior, it is necessary to provide structure that includes both routine and responsibility. Try developing a list that includes social/recreation (video games, get togethers with a friend, research favorite topics, etc.); work/school (household chores, meal planning, cooking, look into academic programs, look into vocational programs or search for a job, volunteer work); and care for self and others (bedtime, wake up time, showering at least 3-5 times weekly, brushing teeth daily, caring for pets, helping others in the family, etc.).
Next work with your son to put the tasks on a calendar and make a daily schedule. If he doesn't want your help, find someone he will listen to with whom he is willing to work. Sometimes getting out of the house for appointments is a good first step and helps provide structure and routine.
Lastly, find things that motivate your son to get out of the house. Will he go out if he wants ice cream? Will he go shopping for favorite foods?
It may also be that home provides too many comforts and he finds it unnecessary to get out and work or attend school. If that is the case, you will need to determine what things are necessary and what things are privileges that must be earned.
The trick is to keep him accountable and to help him find balance, while building tolerance for less preferred activities. At the same time, it is important to find strategies to help him feel encouraged, capable, and appreciated.