I'll try to answer from my personal experience, although in my case it is not the issue of skin color, or relationships of couples playing.
To begin, let me introduce myself, I am French and I was born in Algeria. I currently live in France and I have many friends of North African origin who knows my story. I lived in a country where there were three religions and the religion we all knew each other in the classroom and elsewhere. As a Christian, I was a minority in relation to Muslims. Here in France, it's the opposite. But Muslims are comfortable with me because they know I know a lot about religion. It is not necessary to explain everything to me, a mere hint is enough. Example, one day I was with a craftsman and passing clinic, he just told me he had to take rendes up for his son. Knowing the age of one (6.5 years), I knew it was for circumcision and seeing what I understood, the father alluded to his own, when he added qu'aujourd 'hui is better because it is done under anesthesia (as heard contrary to what he had experienced in his youth).
Now, even if I am accepted in the Muslim community, there are things I will skip the same way a Muslim. I believe I have a duty of confidentiality and I can not tell what some Muslims say them. Like one of my Jewish clients who had treated one day before me, one of his neighbors "dirty kike," I told him he should not say that, she replied that her, she could do because she was Jewish!
So I guess your boyfriend must be in the same situation. He must watch his words for fear of saying something that would be misinterpreted or even he thinks he can be misinterpreted. This is worse, because it is self-censorship. But because of this, it may seem remote for those who do not know him. What can make it even more uncomfortable. Certainly it would may happen to uninhibited and you try to put them at ease, saying that the timidity of your man is due to his fear of saying something stupid. This may clear the air and allow to talk more freely.
Sorry, I may have been a bit long!