I had drafted something you could say to difficult child about this. Certainly, I would recommend role-play with difficult child to help him learn how to handle this sort of interaction in the future.
First, although difficult child was being honest and open, that is not always a good thing and can be misinterpreted (as I feel it was this time). Say to difficult child, "What is your favourite toy?" then explain to him that to someone else, it could seem a stupid choice, or bad for him, or too immature. How would difficult child feel if someone said that, just as he was trying to share with them how happy he was in his toy? Of course difficult child would feel angry and hurt. How would he respond? What would be safe way to respond? Perhaps a good response could be, "I'm sorry you don't like it. That's OK, it's your right to have your own opinion." Then leave the topic. Never try to convince someone to think the way you do, is the lesson for today.
Now go to the situation with neighbour. You be the neighbour, showing off a new toy. How could difficult child have handled his distaste for the noise, while still not offending difficult child neighbour? Perhaps difficult child could say, "It looks like fun, but I don't like loud noises." Or maybe just, "It looks like fun, I prefer quieter toys."
"You" statements are generally perceived as more confrontational. Neighbour's response was a classic offensive "you" statement - "What are you, a girl?" It's equivalent to saying, "You're a wimp!"
Neighbour should have shown more maturity, of course. But if you can show difficult child that not all adults get it right, but it is important to not try to correct other people; be responsible for your own behaviour, not the behaviour of others. And accept, not correct, the lack in others. As we want them to accept the lack in us!
This way you can turn the whole experience into a positive one, that helps difficult child see it in a positive light, see himself as acceptable and accepted (by you) and devalue anything negative from neighbour.
Never lose an opportunity to improve your child's education and self-esteem!
Marg