At just two my son's major preoccupation was "where's mummy/grandma, have I been ABANDONED???"
I couldn't go to the loo at toddler groups without taking him with me. I certainly couldn't leave him alone with anyone but my parents (who have looked after him since he was 10 mths old two days a week).
Now though, it's 6 months later. I notice he is much more confident. The other day he quite happily trotted off with his sister to play at a (admittedly familiar) neighbour's house, waving bye to me and holding the dad's hand.
Noisy situations with lots of kids still give him pause for thought, but he gets stuck in much more quickly now and is not at all worried if I pop to the next room to get some tea etc.
He has also just done potty training and is pretty reliable now plus his speaking is so much more advanced than it was 6 mths ago.
I plan to start him at preschool in September, when he will be 2 and 10 mths.
My view is that very young children are mainly worried about not being left by you, then as they advance towards three they get over that and are much more interested in friends and the external world. Some reach this stage sooner rather than later. My DD started preschool at 2.5, she was definitely ready and there were no tears or anxieties. However my son is a bit later to be ready.
You don't make a very small child sociable by thrusting them into preschool if they are not yet developmentally ready, you just make them anxious and more clingy. In fact I think children are more confident if they are not forced into social situations while they are still in the separation anxiety phase.
If I were you, unless your son is naturally very gregarious and sociable and not anxious about separation from you, I would wait till September and then see how much he has changed in the last 6 months and how much more ready he will be by then.
Good luck