Sponsoring my cousin (over 21) on B1/B2 visa while I am in US in H1B

Question

Hi, I am Bangladeshi working in USA with H1B stil valid for 2 more years. I sponsored my mom for a B1/B2 visa and she got visa in her very first interview. Now, I want to sponsor my cousin too for B1/B2 who is over 21, whi is studying in Dhaka.

What type of documents he need to carry to the interview for himself? Do I have to write a letter as well? If yes, what sort?

GIve me some advice as to how to increase his chance of being successful in the interview? I understand that in my mom's case it was obvious coz she is my mom, but about cousin I heard success is not much. Please advise.

-Farhad