Firstly, what a wonderful woman you are! Has he cleaned up their cum already? If so, then sucking should really not be a problem.
What do you do to give him pleasure sexually?? There is always tye threat of cutting off a pleasure he has, unless he agrees to at least try sucking one cock. If you can remind him how much YOU want to see him sucking BBC, and that he has watched you, so it is only fair.
Maybe start slowly, ask him to feel how heavy one is. Turn him on while he holds it. Then maybe he gets real close as he watches you suck it, and keep stopping to kiss him, then when things have really heated up, just guide his mouth and kiss around that BBC