I have Asperger's myself.
At first, my obscessions drove my poor mother crazy.
they went like this.
Trucks
Aeroplanes
Trains
Video Games
Computers
Mobile Phone Hacking
Musical Equipment
C++ Programming
After a while, she learnt to switch off.. and just says things like "Very Good" or "Thats great!" even though it probably drives her up the wall.
These days though, she is asking me alot about computers :D
The more I think about it, it would be worth you posting this on the ADHD forum. Not that he has ADHD...we really don't know but Sandman2 will answer and he used to be a principal and teacher and he knows a lot about child behavior.
To be honest with you, I'm not a doctor so I am not qualified to give any kind of diagnosis. I will say that the obsessive part of OCD usually revolves around irrational thoughts. Obsessive thoughts about ones health, harming others, catastrophizing thoughts, etc, and not so much an obsession with one thing in particular such as trains, or engines, etc.
The compulsive component revolves around having to do a ritualistic action to alleviate an urge. Such as counting, checking, etc. Not performing this ritual would result in anxiety for this person.
So your question is a tough one for me. If this were my child, I would definitely seek out a professional such as a psychologist that can diagnose his behaviors so that he can receive the proper treatment. I think what he has is an unanswered question mark and that "self-diagnosing" is probably not the best thing to do by either of us. He may not have anything other than a brilliant inquisitive mind.
My best.