The parent of an AS child asked why her son never wanted to do his homework until the last minute. I responded:
I suspect there are many "etiologies" for procrastination. Someone I know who is totally undiagnosed but probably ADD simply cannot do anything (with very few exceptions) unless she is facing a deadline at very close quarters. She can't start to read a library book, for example, until the day before it is due.
In an AS person, the reason is more likely to be related to transitions and motivation. The only time it's easy for me to start doing something is when I truly want to do it (i.e., the motivation is entirely internal) and I feel good (not tired, not stressed, not anxious, etc.). Feeling anything in particular can be very distracting and prevent me from concentrating, even when the project at hand is one for which my internal motivation is strong.
In all other situations (including those where all that's missing is the quality of being entirely self-motivated rather than motivated partly or completely by something in the outside world), there is a "sill" over which I have to push myself before I can start. That takes energy. It's as if my brain is mobility impaired. It can't simply walk casually from one focus to another, it has to prepare, plan, and then propel itself into the new activity with an expenditure of muscle-energy.
Something about the AS "internal economy" (for most of us ASers) seems to have an inbuilt default setting for conservation of energy. We can learn to over-ride that setting (a learning that takes much longer than those around us like to imagine), and it can be over- ridden spontaneously by an uprush of internal motivation (as when we have a chance to focus on a topic of special interest). But for most of our young years, it can feel like a major production to move out of default and into the energy-expensive project of focusing on a project by which we are motivated externally. We therefore may tend to put off making the switch as long as possible.
I don't know whether that is your son's situation,
but I'm sure it is a common experience for many AS
people. It's something I still have to struggle with
at work almost every day. Sometimes I put off easy
chores at work because just thinking about them makes
me feel unbearably tired. (Lack of internal motivation
can be so draining that I actually feel queasy when I
try to force myself to focus on one of these tasks.)
Only when enough time has passed that the chore has
changed (by the passage of time) from mundane to near-
critical can I summon up the motivation I need to
get me going on it.