Trouble making friends at pre-school...

Hello.  I'm normally on pregnancy boards, but I have a 4-year-old son.  I have a question about my son making friends at pre-school.  I might be over reacting, so please bear with me... 

We moved to a new town in April, and my son started going to a pre-school in the neighborhood.  Yesterday was the first day of this school year, and my son was happy to go to school.  When he came home, he said he had fun, so I was happy to hear that.  However, later on, while we were walking our dog, he started telling me that when he walked up to some kids to play together, they ran away from him.  These are kids who he made friends with in the previous year (between April and June), and one of them often comes to our house to play.  I asked him if he had asked them if he could play together, and he said he doesn't think asking would work.  He tried to play with them a few times, but they kept running, and no one would play with him, so he decided to play by himself.  He said, "playing by myself is fun too."  He told me everything in nonchalantly.  I asked him if he felt sad, and he said,"no, maybe a little bit, but I'm fine."  He says, "everyone is my friend, and they are all nice."  When he started this school in April, I went to watch him a couple of times during the outside play time, and I saw some kids refusing to play with him, saw him playing by himself, and some kids trying to take away the toys he was playing with, but I thought it was only because he was a new kid.

Imagining him isolated makes me upset.  I changed schools twice when I was a kid, and one of them was a very bad experience, so I'm having these flash backs of those days, and that doesn't help me either...  When I think about myself, I'm blessed to have some very good friends, but I can't say I'm very sociable, so I'm afraid my son is going to get influenced by me.

My son used to be super shy until he was 2 and a half, like he hated everyone but me.  When he became 2 and a half, something changed inside him, and he became very outgoing.  Even then, he always prefers playing with adults or older kids who care about him and take care of him, and he seems to have harder time to make friends with kids his own age.  Like the boy who comes to play at our house but runs away from my son at school, he has a brother who is two years older than he is, and my son gets along with this brother better.  

What should I tell my son about the kids who run away from him?  He obviously wants to play with them.  Should I tell him to forget about them and play with someone else or by himself?  Or should I tell him to keep trying?  I don't think those boys are bad, but some kids become very mean when they form a group, so I don't know what to tell him.  My son doesn't seem to care about it as much as I do, but as a mother, I'm hoping he has a good time at school...

I'm sorry again if I'm over reacting.  After all, not everyone gets along even in kids' world, so I really hope he will find friends who he can get along very well.

Any opinions or suggestions are appreciated.

Thank you!