F2/B2 visa for graduation ceremony?

Hi All, I am currently in US on F1 visa and working on CPT. I would be continuing the same job on OPT as well after I graduate in Dec 2016. I have applied for F2 visa for my husband in Nov 2016 for Dec and got rejected. I have my graduation ceremony held in May 2017. 1. Which visa do I apply for my husband? Is it F2 or B2? 2. When is the right time to book an appointment for visa interview if the ceremony is in May 2017? 3. What are our chances for my husband to visit me to US? Appreciate your response.