How to help young kids to make friend at school?

Hi,

I guess many parents may had/have same concern as I do. My son is 3 year old now and started to care about who is his friend or not. Yesterday after he arrived hom, he wasn't as talktive as he usually be. Later on, he told me XX(who used to be a friend of my son) said bad word to him. I tried to get some details out from him but he wouldn't want to talk about that.

My son is a gentle boy and always gives me positive comments about just everyone he interacts daily in school (sometime I try to find out how things going by asking him questions like "Do you like Ms XY?", "Whom do you like most at school?", "Do you like this new school, or the previous one?"). So this looks like something that really has hurt him.

I would like to know what the parents here are/were doing to helped your young kids to make and keep friends, develop good personality and characters. Any experience sharing or small tips will be highly appreciated !