首先,你的题应该纠正为
______as he is ,he knows a lot.
A.A young boy B.Young boy
as作“尽管”讲,引导让步状语从句,从句是倒装句,要将表语或状语放在句首.例如：
Proud as these nobles are,he is afraid to see me.这些贵族尽管很傲慢,他却害怕见我.
Hard as he studied,he did not pass the exam.尽管他努力学习,但这次考试他还是没有及格.
Small boy as he is,he knows quite a lot.尽管他还是个小孩子,但是却懂得很多.（注：在这种句子中,作表语的名词前不加冠词.）
所以答案是B.