My husband was J1. In order to satisfy 2-year physical

residence rule, he went back to china, and worked in China for 3 years physically in order to fulfill the J 2-year residence rule.

Now he came back to USA, and is going to submit his 485. USCIS will absolutely need his J waiver evidence. So, how can he prove that he lived in China for 3 years? What kind of document should he submit?

many thanks