How's that for a subject?
DS has always been really independent. He just had no desire to play with other kids, and for the most part, he still does. His imagination/imaginary friends are enough for him. Seriously, he'll tell me that he didn't need to play with anyone b/c he was playing with Nemo/Mickey Mouse/Morty/etc. Every once in a while though, he'll attempt to engage kids. He always seems to pick kids older than him.
Today he decided that he wanted to play with a pair of five year olds. They just missed the cut off for school, so they're still hanging out with the playgroup instead of in kindergarten. I guess he thought he could get them to ride on his imaginary train or something, but they just started yelling at him to stop tooting, stop being weird, and to just be normal. Then they told him that they didn't want to play with him.
He didn't really get it, but I see this happening more and more. He just won't stop playing his way, which I guess is good in some respects, but it gets in his way too. I don't know how to encourage his imagination and also teach him that sometimes it's better to be a little boy instead of a train.

Re: Kids are mean and DS's imagination gets in his way.
Thanks for your reply auntie. This is actually something I've been wondering about for a while. I was lurking on Special Needs a few weeks ago and saw a few of your posts which made me question DS's personality (actions?) enough to look some stuff up and mention it to DH. I strongly suspect that he may lie on that end of the spectrum, but DH isn't ready to talk to the pediatrician about it. Because we've been able to help DS work through some of his sensitivities--he has a lot of problems with smells, sounds, and tactile things--he thinks we might be able to help him socially too. He wants to see how this year of preschool goes and then talk to the pedi at DS's 4 year appointment in early May. I kind of feel like that might be waiting too long, b/c he'll be with the same kids again next year--if they've got a certain impression of him by then, won't it be harder for him to make friends?
DS has lately been attempting to interact with select kids. He's just really bad at it, and he doesn't get that others might not actually want to do the same things that he does. At least, not for 20 minutes straight. He much prefers to just play with adults, probably b/c they humor him and find him amusing.
As far as what he does, he seems to vascillate between being the engineer and being the train. He often calls himself an engineer but acts just like a train. Or he calls himself an ATV driver but at the same time says he's a purple ATV. He tends to get stuck on topics for 4-6 months at a time. This time last year, he liked to pretend that he was Frank the Combine from Cars. That stuck around until a little after his birthday in April. The play does tend to be the same every time.
I agree. We have a range of kids in our tight-knit neighborhood and the 5 year olds were very rude and snooty about letting my 3yo play this summer. The 7 year olds weren't a problem and the 6 year olds were starting to realize being a turd isn't necessary. Hang in there and cherish his uniqueness! You probably do, but you can specifically talk him through failed interactions after the fact. Kinds being mean because he "tooted" or something can be addressed with "Those kids were not acting nice. When kids say mean things, or act like that you can always find a different kid to play with." If they shunned him because of his imaginary play, you can talk him through how to engage kids in ways that won't put them off. Each situation is unique and will be addressed differently, but some kids (and big people) need explicit instruction on how to handle social situations. The more you teach, the more he'll learn. Your heart must break when kids act like that. I know mine would!
OP - is your kid in any preschool setting? I would be curious what the teachers say about his behavior.
Ditto PP, I'd get this checked out now if you have suspicions. My BFF's son has mild Asperger's that was evident from 2.5 onward (especially the imaginary friends and singular fixation on topics for long periods, strong preference for adult interaction). I, and others, tried tried to gently express our concerns, but BFF and her DH chalked it up to him being an only child and at home w/ mom all day. When he started preschool, the teachers suggested he might have some Asperger's/Autism tendencies and again they blew it off and thought he was just immature. Finally in KG, the school insisted he be evalutated and he was DX w/ Aspergers and ADD. He was enrolled in a therapeutic playgroup and had an IEP w/ school from K-4th grade. He's in 6th grade now, w/o and IEP, but he struggles w/ interpersonal relationships and his parents spend alot of time coaching him on social interactions and peer relationships. He does have friends now, but some of the issues he had in earlier grades could have been mitigated if he'd been evaluated earlier and had a chance to form some stronger peer relationships when he was 3 and 4.
I just want to piggy back a bit on your conversations with Auntie. While I totally get the wait until he's 4 to talk to his pedi and then go from there it is important to realize that getting into experts and started in therapy is a very different process than our own going to the doc. I initiated EI for DS #1 when he was 22 months and it took us 4 months until he actually had his first therapy session.
In your shoes I would find a good list of how Autism is diagnosed and take a good look at where you think your son has parallels and then find the Developmental Pedi's in your area that you would be interested in evaluating him if needed and find out what kind of wait you are looking at. You may feel differently, your husband as well, if you find out that he wouldn't get in to see a Dev Pedi for 6-9 months + after you made the appointment. If that is the case then he could easily be 5 before you're able to get him started in therapy.
Again, I'm not saying you'll even need all of those things but it wouldn't hurt to have all the info to make the decisions.