It is about 10 months since the boy has__ a young pioneer.A

一般这个固定句式应该是这样的：It is about 10 months since the boy became a young pioneer.
表示“自从……以来已经过了多久”
since 连词,后面接过去的某一个具体的时间.
例如：He has lived here since 1998.他从1998年就住在这里了.
如果接句子的话,要用一般过去时,指“自从某个动作发生之后,（主句）”,而且这个动作应该是短暂的、瞬时性的.
比如：I have been in love with him since we two first met.自从我们第一次见面之后,我就爱上了他.(...>.