i remember feeling that way in school... and i still do. does he normally have social issues? is he normally shy around new experiences and new people? this is most likely totally normal, your child might just warm to the new situation a little slower than other children, and i wouldnt worry about him too much... however... i would like to tell you a story about my experience....
dont let this freak you out, but my mother always thought that i had high functioning autism. aspergers. basically, very little social skill (especially with kids my own age) but works ok with adults and teachers. usually aspergers kids are very smart! :D also usually has an obsession over one thing. when i was young, and still today, i LOVE LOVE LOVE pretty rocks. during high school i was obsessed with a teacher of mine. LOL. didnt realize until i was about 22 that it was most likely a symptom.
ive never been diagnosed, but the more i learn about autism, and the spectrum itself, through training related to my child care, the more i see myself as an aspergers person.
however, it never really crippled me, i was never treated like i had a disability, and i live a happy, fulfilling life, even though i still to this day have a very hard time socializing... but like i said, im not crippled. i have a loving husband, a child, friends, family... im not a shut in at all... i have a life. it just may be a little selective about it.
however, now that ive probably scared your pants off because i mentioned the A-word.... i want you to know that its unlikely that this is the reason for your child's problems. however, even if it is, its not really a problem. i have discovered that i am very choosy about who i talk to, and who i am friends with, and even though that narrows down my "options" i find that the friends i have in my life are pretty high quality for my life. LOL. you know what i mean? i cant just be friends with just anybody. i have had strong frienships, for my whole life.
so anyway, you could have him "tested" to see if this is an autism issue, remember that even if it is, it sounds very slight, and may only give him slight problems socializing. or you could just leave it alone, and let him be the way he is. just support his feelings, let him have those feelings, and as long as he does well in school, and isnt misbehaving either there or home, he should be fine. it doesnt sound like he is having a terrible time, he just probably doesnt know what to say, or who to talk to, or what the kids will say to him, or if they will pick on him.... you know? theres a lot of anxiety to talking to people.
if you are a people person, you might have a hard time understanding how he feels, but dont ever ever pressure him to socialize or make friends... he will in his own time and they will mean a lot to him. just have a lot of love and patience for him, and he wont feel that its bad to have the feelings he has. :D
good luck and dont worry - autism is NOT a death sentance, especially cuz it sounds like your son is a smart, healthy kid! just might have a few social issues. after all, they say now that ADHD and that are autism spectrum diseases. no one eevn thinks of those as being serious. thats what we should think about autism in general, especially the high functioning side of it. :D have hope, its not a bad thing! :D