K.
This is a phase, it will probably happen again between 2 1/2 and 3. What I have found is that after about 30 minutes out of site the kids settle down and forget they miss you. Of course as soon as they see you it starts all over again. The best thing is to use the daycare at the YMCA, babysitters- as long as they understand about the problem- so they are prepared. The more often your son is exposed to dealing with other adults (or sitters) than realitives the easier it gets. It is said it is actually harder on the parents than the kids.
Don't forget the phase may come back again before your son is ready for kindergarden. Just work through it the same way.
Lots of luck,
K.