never experienced BBC

How about talking dirty during sex and see how turned on his is. Do some role playing with threesomes that have bbc involved. If he is open to this then start dropping hints at other times. If he is responsive then start openly talking about any fantasies he has and bring them into play. If he goes for it talk about your fantasies and maybe he will be open to them. Does he like toys. Change the color of the toys maybe to brown then black. If he like to play with them with you then lay all three out for him and let him choose. If he starts using the black one more than others then ask him if he would like to see it for real. Then talk about the idea and be open. Maybe these are some helpful ideas