Book for teaching 6yo age-appropriate friendship/social skills?

Does anyone have any recommendations for teaching a 6-year-old basic friendship/social skills?

DS is almost 6 and I’m afraid he’s on a road to alienating the few potential friends he might have in our very small community. For example, today we were at the public school while little brother was having his speech therapy. We stumbled upon the kindergarten class getting ready for recess in the hall. A couple of them excitedly called out, “Hi Kai!” What did Kai do? He scowled, put his head down, and hid behind me as we walked by them. So I greeted the kids myself. When I asked Kai why he didn’s say hi back, he said he was feeling shy….but still….I’m sure those kids will think twice before greeting him again and then facing that sort of perceived rejection from him.

I’ve seen similar areas where his social skills need refining too….like being bossy regarding whose turn is next in bowling and trying to grab away a bowling ball from a boy when it was Kai’s turn to bowl and the other balls were mysteriously missing. Really, all he would have needed to do was ask the boy for the ball. I know it’s typical 6-year-old stuff, but it’s problematic in a small town where everyone else is in the public school.

At this point, all I can do is talk with him about the incidents after they happen….but I’m not always with him in social situations and am afraid there is a lot I’m missing that will not be to his friendship advantage. He’s fairly good about following through on appropriate social behavior if we can anticipate what’s going to happen and practice, but that’s hard for me to be on top of. That’s why I was thinking a book that covered some of these things of how to be a good friend or make a good friend would be helpful.