A.W.
Hey S.,
Personally, I think its a good thing that your son is naturally cautious with adults he doesn't know. I see ho this is going to pose a problem soon - but I do think its good.
Do you know where he is going to preschool yet? I taught preschool for a few years and I'm a big fan of meeting shy students a few times before they begin classes at the school. If you know for sure where he will be going, maybe you could find out which room he will be in and talk to his future teacher - I've had lots of moms do that before.
Its been my experience that the mom would call my room or even email me and explain to me that her child is very shy or has never been to school or daycare before - and we would work out a few times to meet either at school or at a park or something, and have just an hour or so of face-to-face time with Mom there... it made transitioning a lot smoother for most.
Just a suggestion. Also - once he does get into preschool and gets used to his teacher, he may mellow out some. Especially if you were to volunteer occasionally and let him see you and his teacher chatting and what not.
Good luck!