Sentance a. is worng.
Verbs after verb to be (am, is, are, were, was), has to add -ing.
You can say : He was coming from China. (Past continuous) or
He is coming from China.(Present continuous)
Sentance b. is right.
He came from China. (Past tense) or
He comes from China (Present tense)
However, the meaning of a. and b. are different.
The meaning of a. will be: He is in China at the moment and he is coming back.
The meaning of b. will be: He is Chinese. He is from China.