My son cries and is upset by almost everything when he plays with other children. Granted, they're always pushing him around, pulling on his clothes and hair, taking things out of his hands, etc, but isn't that all kids his age do?
Other kids don't seem quite so reactive, and I think we make other parents feel bad. Their child can do the slightest thing like pull on his shirt and he turns red and starts bawling, then when they discipline their child, that makes him cry even more and he comes back to me and doesn't want to play with other kids anymore.
When they first meet, he's interested in the other children but once they upset him, he prefers to just turn his back and play by himself.
Is it a confidence thing? When we're at home or at the grandparents', he's roughed up by our dogs and they don't upset him, it just seems to be people and places he's not really familiar with.
Is there anything I can do to make him less sensitive to other kids or be a little more resilient?
It's very early, he likely just would rather be held and cuddled by you at this age. You are his stability, his protection and world. There will be so much time once he is older to interact with children of all ages. At this age though, mother is largely an infant's world and buffer against all that he feels vulnerable to. I would just respond to his needs, and when he is older he will feel safe and secure and respond more comfortably to the interactions.
My Little Miss was (and still is, to some extent) easily upset and frightened by other children's rowdy and rambunctious behavior. Since rowdy and rambunctious is pretty normal, I mainly stuck close to her and we played with her on my lap for a while. If another baby came on over to investigate, I'd do the majority of the interacting so I could head off any really aggressive behavior that might send mine into hysterics, and also so Little Miss could see what was expected when another human walks up to you with a smile on their face and a toy in their hand.
I did curtail one relationship with a mother/daughter pair because the daughter was very, very boisterous; when she would run up to my daughter and scream and jump the mother never ever did anything to model calmer behavior for her daughter, although my daughter would shake and cry without fail. The mother also accidentally hurt my daughter, and didn't seem to care, actually saying it was fine. Obviously it was an accident, but it's not fine, please pick my poor kid up and apologize for your mistake! So it wasn't the fact that my kid couldn't handle the other kid, it was that I noticed that this pair found their behavior perfectly acceptable at all times, no matter what the consequences. Little Miss still has plenty of little friends that make her cry, but the other mothers are right there with me, talking the kids through the interaction in a way so no one feels rejected and no one feels consistently terrified. As an adult, if someone is just "too much" for me, I'll maybe tell them they're a bit overwhelming, but try to do so in a way that makes them feel that I like them and want to be around them, or I'll just cut down on contact. You can model both behaviors for your sweetling, although you can't expect anything to sink in for years and years yet.
And on 09/23/2011, we were three; husband, daughter, and me!