This is a tough one and I see your dilemma.
Personally, if I saw a 6yo boy walking down the street in a dress I'd want to high five him an his parents for being brave enough to be himself in a world where people are quick to judge and bully.
But, I totally see where you're coming from in regards to bullying. As PP in this thread have proven, some people are still narrow minded and don't see that boys can wear dresses and love pink/purple and think these things are just reserved for girls. It's such a shame that we can't just allow our kids to be themselves for fear of them getting hurt
You're a great mum for worrying that he might get hurt