Bullying is about perception, kids can be as nasty as they like, if the nastiness has no desired effect then they're just failed bullies. It may be the case that your child is not a victim of bullying but rather a ridiculous curriculum (yes, they do teach bullying in school now days) which has encouraged him to be over sensitive. Or it may be the case that the other children are bullying him and thus is merely the tip of the iceberg. Or you may have an in between where some kids are not reply because they don't like him. Either way a bit of social problems are character building,just make sure that he learns to take it in his stride.

Unfortunately, almost everything is classified as bullying today. I'm concerned this minimizes genuine bullying, plus reduces the coping skills kids will need to develop for living in a world where not everyone is nice to you.

depending on his age, he could have made that up so you'd leave him alone.

He better not work in retail.

If he's older than four you failed as a parent. L

Help him learn not to read anything into that. Be kind and courteous because that's who you are but do not expect it back. Do it because that's the type of person you want to be and no other reason. No one owes anyone pleasantries. Not to mention a variety of social issues could be at play on behalf of the greeter and/or the greeted.

Seriously this is an fml, people I have worked with for years don't say hi oh and grow the hell up people are shy.

If the kids not saying hi back are doing so in an intentional method and pressuring others to likewise ignore him, or they start ignoring him during class activities that IS a form of bullying. It's called "unpersoning," or social isolation depending on the severity. So it may not be bullying yet, but it could be the starting stages of a very harmful form of bullying.