Etiquette for Introvert Kids

How can I get my kid to interact with others with thoughtfulness?  My 8 year-old son is an introvert yet doesn't seem to have any social anxiety.  His favorite activity is spending time at home reading and he rarely asks to get together with friends, yet he's comfortable going most places and trying new things.  He fits in at school, church, and other community places.  He can make quick transitions from one activity to the next.  He can talk to adults and strangers.  In settings where he knows everyone well and is interested in what is currently going on, he will be animated and even chatty.  He may see those same people a few hours later, however, and act like they are invisible.  Since he was a toddler, I've noticed that he really doesn't give social interaction much thought.  If people ask him questions and he's not interested in the conversation, he'll just give a deadpan look and not respond.  If we walk through the playground or school ground, kids will call out to him to say hello but he will only return the greeting if nudged to do so...and his reaction is then minimal and impatient.  I want to honor my son's nature.  I know he's never going to be terribly social.  But he comes across as rude and I see how he hurts others' feelings.  He's an only child and I want to teach him to how to build and nurture a caring network.  I'm fine with that network being small, but I'm not fine with my son blowing people off.  I honestly try not to nag, but my husband and I been working with our son on please, thank you, eye contact, and non-flat attitude for years now without much change.  My husband and I are not perfect, but we respond to others with warmth and are polite to everyone (including each other), so we model what we're asking for.  Ideas?