If you are serious about cuckolding him. You need to slow it down and go very slowly. The first thing I would do is not bring it up again. You may want to start by picking out a larger bbc dildo and try to incorporate into your lovemaking with him. Start using it little by little each time. Get him used to fucking you less and less. While you ask for the dildo more, start teasing him lightly about his penis size. If he doesn’t have a small penis humiliation fetish, constant teasing him about his size will give him one. Keep him on edge, don’t let him come till you do. He will start to associate that better sex with the bbc dildo and not his cock. The small penis humiliation will hammer that idea home in his head. Once you cum on Bbc dildo, jerk him off teasing him about his size and inadequate cock. He will cum so hard from it. Do this over and over again in bed eventually he will be begging you to fuck bbc while he jerks all over himself