“因为他在读初中一年级”英语怎么说

because he is in Grade 1 in middle school.

because he is in six grade (美国6年级是初一)
或者
because he is a freshman in junior high school. (意思是初中新生也就是初中一年级.)
美国初中高中大学都是4年的,所以freshman是指第一年的新生接着是sophomore, junior和senior

就直接说：Because he is in grade one of junior school.

Because he is now in middle school, Grade 1.

because he is in Junior 1.
简洁又地道~

for he is studying in grade 1 in middle school