Oh poor you - I was exactly where you are about 18 months ago (when ds had recently turned 3) - is he/isn't he? Lots can be explained away by being a boy/just quirky/but other children do that too/he'll grow out of it/just shy etc etc.
Like you, I wasn't exactly in denial about my ds's behaviour/development, but it was only when his keyworker at his nursery talked to me that I realised that, regardless of what a diagnosis/label would be, he needed some help. The HV at his 2-year check had also asked a few questions which of course in hindsight were leading - but she was spectacularly crap and didn't follow anything up, so I continued to bury my head in the sand and make excuses for him.
None of us can say whether he is or isn't, of course. But if a (trusted) member of nursery staff has raised concerns, which you kind of knew anyway, it is probably worth following up. Talk to HV and ask for referral to developmental paediatrician. This will almost certailnly take a long time, so in the meantime look into ways of helping him the areas which he is having most problems.
IMHO it sounds like his language/understaning might be causing some difficulties - looking blank at questions, not sure how to converse, echolalia etc. This will certainly be having an impact on his interaction. This doesn't mean straightforward language delay exactly, but social communication, which is a different thing.
There is light at the end of the tunnel, though, honestly. The more I write the more actually he sounds like my ds at that age. He was dx with ASD a year ago at 3.6. We started ABA (Verbal Behaviour) before he was diagnosed as we had accepted he needed help. A year later he is unrecognisable - at the time he had quite seriously impaired interaction esp with peers, poor play skills (ie imaginative play), no friends, unable to have proper conversations, tantrums, obsessive, ultra-controlling behaviour. He is now extremely chatty never shuts up in a sort of normal way if a bit quirky, has amazing and genuine friendships, plays with other children, is compliant (mostly), has brilliant imaginative play, and though still has some difficulties (anxiety mainly) tbh might not even get a diagnosis now.
The best thing you can do is look at exactly what his difficulties are, and how to help these. IM(VH)O he might certainly benefit from ABA (Verbal Behaviour) so it would be worth looking into this. I'm sure others will have more suggestions too.
Worth looking through old threads on this board too.
Sorry for long reply! HTH.