Why are you 'sick with worry', exactly, worried? If your boy does have an autism spectrum condition, he will still be the wonderful little boy that you know and love, with all his quirks and uniqueness - some of which may be related to an ASC, and some not. If he does not have an ASC, then he will still have his challenges to overcome in life, as do we all. He will still be at risk of being bullied, of having mental health problems, of not fulfilling his potential in some way, or of his life not panning out quite how you'd hoped. Just as every child is, regardless of autism! None or some of those things may happen, just as with every child, and you will deal with them as and if they do happen.
Nobody on here can tell you whether your boy is autistic. If you have concerns, flag them up to a professional, and keep flagging them up. They may be something, they may be nothing. Nothing about what you've said screams autism to me (in my limited, personal experience), but if you have a feeling that he might be, then listen to that intuition and keep an eye on him. I had a feeling from very early on that DD had Asperger's. I hummed and hahed for years about whether to get her assessed, and posted a couple of threads on here trying to figure it out, too. I was given really good advice... to stop fretting about it and just get her assessed! 
I did, and she was diagnosed with Asperger's aged 9. The more I read about it, and about how it often presents quite differently in girls (and some boys), the more things fell into place for me. Eventually I asked to be assessed myself, and was diagnosed with Asperger's too. There was nothing to be 'worried sick' about - it has given us greater self knowledge and tools for understanding ourselves. 
Incidentally, early, precocious speech can be a feature in Asperger's, whereas delayed speech is more associated with other ASCs. My DD, though, was quite delayed with her speech, yet still diagnosed with Asperger's. Once out of toddlerhood, though, she became very articulate. I'm just trying to illustrate that thses things are complex, and a good clinician looks at the whole pattern of presentation, not just certain features. The imaginative play thing threw me for quite a while, too - she always had a very rich imaginative world, and would play (usually alone) for hours with her toys in imaginative scenarios. The 'lack of imagination' impairment refers to social imagination, not necessarily the kind of imagination you use in play, or in writing fiction (DD's special interest is writing fantasy fiction; since the age of 8, she has been a prolific and talented writer). She is also very articulate, polite and communicates well. Doesn't mean she doesn't have AS!
Understanding and imagining what others were feeling in a social situation, however, was a lot more challenging for her, as was reading the reactions and body language of other children - that is the social imagination part.
If you want to know more, I'd recommend Tony Atwood's book 'Asperger's Syndrome'. The clinician who diagnosed DD referred to it as their 'bible' (mainly in terms of diagnosing girls).