Kids are kids, they aren't born knowing everything.
By shhhing your son, you kind of made a big deal out of something that's not a big deal.
My 8 year old, when she was in Kindergarten, was the only white kid in her class the first half of the year. All the other kids were either black or Mexican. (And yeah, they're 'black', they are too young to care to call themselves 'African Amercians', it's very literally 'black, white, brown, yellow' at that age, LOL)... anyway, they were soooo confused by my daughters 'white' hair (she's so blonde it's white)... no big deal. At that age, the black kids think blonde hair and blue eyes is different and cool ;)
There's really no explaining that needs to be done, other than 'everyone is different, and that's perfectly okay'. As long as kids aren't slinging racial slurs, curiosity is nothing to be embarrassed about, nothing to 'shhh' ;)
Same goes for when kids see someone missing an arm or leg. They're just different. Someone doesn't speak English? They're just different. Someone wears glasses or has a hearing aid? They're just different. No big deal ;)
It's our differences that make us unique!
I used to babysit these kids from Ghana, who were so black they were blue. My kids are blonde with blue eyes, super white. People used to ask me all the time 'Are they all yours?' and I'd point to my kids and say 'oh I babysit them'... the 8 year old I babysat, Megan, thought that was soooo funny, and she'd say 'yeah, don't I look JUST LIKE my mom?' and hug me. Ooooh they looks we got, LOL!!