Your son sounds very much like mine, and I don't think mine is at all likely to be on the autistic spectrum. I'll explain why in the course of drawing some comparisons with yours, so this may sound like a paean to my DS, but I think it's important not to overlook the good stuff.
Mine is very sensitive to hoovers, hairdryers, handdryers in public toilets, things that beep in hospitals, etc. We had to stay in hospital for a week after his birth and all the staff commented that he was extremely alert and very sensitive to his surroundings, especially noise (e.g. unpleasantly startled if spoon dropped in distant cubicle). But he started to smile, make eye contact and laugh very early. He had real tears from the off.
He tries to wrangle guarantees out of me that no children will be there before we go to the playground. He has always, even since being a newborn, screamed his head off if I tried to take him to an NCT coffee where there were other babies. I've had to take him out of baby swimming, baby drama, baby singing, baby everything, after only a few terrible experiences (for me and the other parents) at each. Like you, I do persist in trying to socialise him with playgroups etc., but it no longer bothers me that we have to leave early when it's clear to him he's expected to take part in anything organised. I just trust him, as he is genuinely upset if forced into a social role, and there's really no benefit to him developmentally in this; he'll have plenty of time, and doesn't have to be the perfect team player pre-school. He has reached an accommodation with other children, but does see them as a different species. He's kind by nature, but now knows that they are liable to want to play with all the same stuff as him. There is the occasional toddler he gets on with just about, but his preference would always be for adults.
He has started to get worried about mess on his hands and want to wash them much more. This is a phase I've seen other toddlers get to at his age, and I don't think it means anything. I leave a tub of water next to the sandpit so he can clean his hands and his toys. He doesn't want to do fingerpainting, but loves painting with a roller or a thick brush. He adores splashing bathwater all over us.
My DS is bright too, in similar ways to yours. He is also very emotionally expressive, very loving and very sensitive to my feelings, particularly the ones I've tried not to show him. (Overheard by my DP just after I got some bad news the other day and thought I was acting normally: DS going downstairs, making me a pretend meal and saying: "Mummy, you alright? Don't worry, Mummy. Here breakfast".) He loves to play peekaboo, do animal noises, turn himself and the rest of the family into the robot/duck/penguin family with appropriate noises/body actions. He has a special growly voice he uses when he's pretending to be a builder and fix things (generally by banging them very hard). He has been into imaginary play since he was about 15 months old and loves giving his animals tea, tucking them into bed, etc. He interacts well with adults he trusts. He is very inventive and makes up in-jokes and has us in stitches. So I have absolutely no worries there.
What gave me a lot of confidence in him from early on was googling all of this and discovering that a proportion of children just have this personality type. It has been labelled as "highly sensitive child" ("HSC" for short if you're putting in a search term) and you'll find lots of references on MN to parents who have an HSC (some from me, I imagine). It does not mean that your child is autistic. Children like this do have a tendency to be bright (though they need the right educational setting to let them be themselves or they can get crushed because they're pretty emotional and can take criticism badly). They're often stubborn and individual and seem to like being this way, and may seem demanding and difficult to others, but that is just part of the package of being overall a very charming, kind, empathetic little person.
I would highly recommend reading "The Highly Sensitive Child" by Elaine N. Arun, if only because it will help you find the patience to just let him be himself, and the confidence to like him without worrying about his not being "normal" as his personality unfolds with age. It also gives tips about what you could do instead of doing the stuff that works with other people's children, which is particularly important at this age as everyone around you starts doing things based around rewards and punishments, which because of your DS's sensitivity and intelligence may be a blunt instrument.