Just judging by the photo I don't even think they're really making fun of him. Maybe playful teasing, but not trying to insult him.

To them that's one of the distinguishing phenotypes they happen to see when they come across an Asian person who they have never seen anything like before in their lives. That's just what happens when people who have never been exposed to something different come across it for the first time in their lives.

Maybe they are trying to be rude, kids can be assholes sometimes, but you can't just assume based on the picture. But anyone who would go somewhere where the people there have never seen anything like them should know "Hey, the locals are going to treat me differently to begin with, and that it's probably not a matter of disrespect but a matter of curiosity stemming from lack of information."