@Heaven12,
I have been black all of my life, and hosting on Airbnb for the past 4 years. All of the issues that I've encountered with the platform definitely have occurred to other hosts regardless of their race, ethnicity, sex, gender identity, physical and mental capability, health status, political views or age. The color of currency is the only one they are concerned about. It is a global company that is always looking for opportunities to expand its market share in every corner of the world. The customer representatives, who are predominantly people of color, are not spending time looking through the photos of millions of host applicants to single out dark-skinned people for rejection. If that was the case, then they would be excluding most of the Caribbean, African countries, Brazil (which has the second largest population of Africans outside of the African continent), Central and South American countries, Southern Asian countries, Polynesian and Pacific Island countries, and many properties in other countries.
It sounds like Airbnb did a systematic check on the property address, and your boyfriend was not listed as the owner. So, the listing was automatically flagged as fraudulent. Most of the customer service reps are not very well trained, and are not familiar with the various policies and exceptions in different states, provinces, territories and countries.
If you've not done so already, I recommend trying to list on other platforms, like Vrbo. If successful, then he will have at least one avenue to market the property. If the listing is rejected for a similar reason, you will have confirmation that the subletting agreement is a barrier, and will have to seek a workaround.