wearing a blue T-shirt and a pair of white shorts 是现在分词作定语修饰the boy.
Do you know the boy?已经是一个完整句.如果再加is,主干句就出现两个谓语动词,句子出现语法错误.
Do you know the boy wearing a blue T-shirt and a pair of white shorts?也相当于
Do you know the boy (who is) wearing a blue T-shirt and a pair of white shorts?
这里who引导的定语从句修饰the boy.主干句就不会出现两个谓语动词了.