My son, aged nearly 20, has just decided to defer his start…

Hello and thank you for using JustAnswer Health for your question. Since you are not on line currently, I have switched this away from the chat form of communication. You are more than welcome to write back and forth if needed, but I thought I start with a discussion and some ideas on resources for you.
The major, final developmental task of adolescence comes in late adolescent or early adulthood and is that of achieving emotional independence from parents and other adults. Developmental task are considered "crises" because if an individual has not completed the minor developmental tasks leading up to the major ones, then these issues can prevent further development, until they are resolved.
If your son potentially has some delay in his social skill development, as well as a learning disorder, these are likely the main areas where he needs more support and direction. Additionally, some young people at this age need a little more time in an unstructured environment to figure out more about what they want to do with their lives. Has your son taken any classes part time at a local college where he can still return home at night? He might be a good candidate for this sort of thing while also getting involved in some type of area he enjoys even if it has been a hobby, while being prompted to think about how he could turn these area's into something that would help to support himself in the future. He could be encouraged to work part time or volunteer in any area that he has a strong desire to have a future in. This should help to motivate him to learn what he will need to do in order to achieve this career.
If he has little motivation to follow through with the above, then seeing an Adolescent Psychiatrist, and/or a Clinical Psychologist would be beneficial. These clinicians have training in diagnosis and could assess and test your son to determine whether he needs specialized treatment or even medication. A therapist will also be assigned - a team approach like this is usually most beneficial as they can assign him activities to improve his social skills, and help him delineate personal goals that he is cable of achieving so that he can move forward. He will be assessed for personality and mood disorders as well. With a group approach like this, you will be involved, while your son will have some sessions on his own.
As far as symptoms of Aspergers, *lack of concern* about social interaction (rather than difficulty with social interaction) is typical of Aspergers . Difficulty with change in routine can be seen with with Aspergers (and Autism) and in OCD (Obssessive compulsive disorder. Of course, to a mild degree it may just be a personality characteristic as well.
As to your question about whether he can be taught how to cope with change, yes there are ways to help with this issue.
Here is a short page for you discussing this (coping with change). It speaks of children, but the principles are good for any age:
http://georgia4h.org/omk/edresources/KidsAndChange.pdf
A good resource to learn more about learning disabilities as well as autism, is this not-for-profit website:
http://helpguide.org/mental/learning_disabilities.htm
I hope that this is helpful and answers at least some of your question, and again please feel free to write back with any thing that needs clarification or any other questions on this topic that you have.