It is a huge adjustment. There are kids who have been in daycare almost their whole life and still cry the first few days. It is a new place/room, new people, and can be overwhelming. Keep encouraging your child, it will get better for your precious boy.
Maybe try to teach him how to approach a kid during play/recess time.. role play "Hi my name is _____. Would you like to play with me? What do you like to play? Can I play with you?" Let him know if the other child says no that it is ok and try to find someone else to play with... usually at least one kid will be open to play.
If you get to drop your child off in the classroom stick around for just a few minutes, if possible. Point out something that other kids are doing/playing with that your child likes and encourage him to go over and play with the other kids. After a minute or two give him a big hug and kiss, tell him you love him, let him know you will be back to pick him up at a certian time in a certian place and leave. Do not turn around, or wait around after you say bye, this will help your child transition and learn to trust the teacher who he is with.
Do you know if there is a clock in his classroom? If so teach your child how to tell time on that type of clock. Maybe even draw on a piece of paper the important times, like school starts, snack time/lunch time, pick up time that he can keep in his pocket. This gives him a better sense of control in a new enviroment, it might help ease his fears a little bit.
Also talk with the teacher and see if there are any other things that you can do to help him. Many teachers have great ideas on how to help lesson the fear of school. Ask her if he is hanging out with certian kids and then you can talk about what your son did with those kids during the day.