I have trouble both with change in routine and transitioning from one activity to another. Lots of Aspies hate being interrupted in the middle of something without warning. Everybody has interests and hobbies that they greatly enjoy and are passionate about, but with neurotypicals, it is only a side interest, something that can be forgotten about when need be. A typical childhood interest, to me, would consist of really liking something as a hobby. But that's it. An Aspie interest is a true obsession. Our passion for our obsessions is so incredibly intense that it cannot be compared with a neurotypical child's "passion" for their interests.
For an Aspie, our obsession(s) is/are literally all we think about, all we talk about, and our lives literally revolve around it/them. To give an example, for your son's interest in Thomas the Tank Engine, if he had to have lots of merchandise, carry around the toys, have Thomas the Tank Engine books read at bedtime each night, etc., I'd qualify this as a normal childhood interest. However, if he spent every possible minute engaging in something Thomas the Tank Engine-related, if he constantly recited statistics and/or quotes about Thomas the Tank Engine, or if he seemed to always tried to monopolize the conversation so it would be centered on Thomas the Tank Engine (including finding any way possible to introduce it into the conversation and reverting right back to the subject if somebody else moved on to a new topic), this leans far more heavily towards becoming an Asperger's "special interest."
The last part is really key. Droning "monologues" about "special interests" is very, very common in Asperger's. Our "special interest" is the first thing we talk about when we see you, and it's the last thing we mention before we leave. This is due to us having trouble with "give-and-take," reciprocal conversations. Us droning, rambling Aspies tend to talk AT people, not talk WITH them. A person is merely an audience for us to relay facts about our "special interest." We really could care less about the social interaction. We want you to sit and listen to what WE have to say. And this is why we quickly revert back to our "special interest," if an attempt is made to change the subject. So, I'd say that it is also these social deficits, in addition to the hyper-intensity of the interest, that sets an Aspie's obsessive fixation apart from a neurotypical child's hobby. Hope this helped!
-GatsbyLuvr1920-