my nephew said something inappropriate to my son. what to do? (update)

Earlier today me and my 4 year old son went to my moms house. My 5 year nephew also lives there with my brother and his wife. Well my son and my nephew were outside playing and I was watching from the door, then my son ran inside and said "mama, Christian (my nephew) told me he was gonna lick my ding dong! And he said it would be yummy" and my son also said that my nephew tried to lick his penis which he calls it ding dong. I didn't know what to say!! I told my husband what happened and he is mad. My son has never said anything like that before, ever! I told my son  to never repeat that and it isn't nice and if it happens again he needs to tell me or someone else ASAP. I told my mom what my nephew said since she was the one who was watching him... My mom only said "oh he probably heard it from a rap song" umm... And he didn't get in trouble or anything. I don't know what to do. I don't want my son around his cousin anymore unsupervised!