I'm not sure I can be any help as I have a DS7 who IS diagnosed with ASD, but I don't agree with the diagnosis. So nothing clear cut is going to come from me! But I can share our experience.
DS has always been the sort to get really into one thing or sometimes a few things at a time. When he was an infant he wanted the same favorite books over and over. Near 1 he was into colors and shapes, after that construction vehicles (and he knew everything about them, not just the type), around 18 months it was numbers and letters, just before 2 he learned all of the frogs and butterflies we could find books on. After that he was into learning to read, but once he learned he was no longer interested, he didn't sit down and really enjoy reading until about 2 years later (although he still loved listening to us read)! We went through Lego obsession and marble runs and mazes (he loved to do them, but mostly he enjoyed designing them). And around 4 he switched gears from being more into words and letters and got really into math. He carried his math workbook with him everywhere for about a year and then promptly lost all interest in math again for another year (but he got back into math again in a big way and still loves it).
He always was able to easily remember numbers and seems to place more importance on them than pretty much anyone else would. We can watch a movie together and later he will say something like "I like that movie, there are 6 scenes with a cat in them." He will also say things like "I want to watch movie x because it's only 108 minutes long and we have an appointment in two hours." I'm wondering how or why he knows the length of the movie, lol. Just today he told me that on the 4th day of March something or another (it was in the car, I didn't fully hear him) happened on the cosmic calendar and I realized he was talking about Cosmos. Now, I remember the cosmic calendar and I remember about when things happened on it's representation of the course of world history, but I would never have remembered the exact dates they mentioned on the show like DS does. But then numbers don't have all that much meaning to me and are not an area of interest.
Right now DS is really into geography and wants to constantly quiz us on info on his maps. He also spends a lot of time just looking at them and talking about capitals and such. He has a talking globe and he loves to listen to the distance between places, while I prefer to listen to the music from the different countries! I think that makes me more normal
DS is in no way NT though, he has ADHD and TS and is gifted (and is too close to call on ASD), so he won't fall into a neat group for compare and contrast purposes. He is doing great though and socially he has come a long way since starting treatment for ADHD.
I will say that I started voicing concerns to our pediatrician about DS at 10 months and we were told not to worry (well, technically I was concerned since he was born as he cried more often than not for about 14 months straight). At our checkup at 2 I mentioned DS was lining things up. That got us an evaluation at 2.5 and an ASD diagnosis. I mean, my kid was weird. He did all sorts of things that concerned us and amazed us, but there was no alarm at the pediatrician until we mentioned that one behavior. He continued to line things up until 3, then the behavior was gone. My point is, ASD or not, something was clearly wrong with DS, but the pediatrician was no help. I did not realize it at the time, but we could have contacted early intervention at any time, we didn't have to wait for our pediatrician to refer us or anything. I would definitely suggest that you see a developmental pediatrician if you are concerned. Maybe they tell you it's nothing and you can stop worrying, or maybe they tell you it's something and you can get your DS help now while he is still young.
