How old is the cousin? Talk to the cousin's parents, don't let your son alone with the cousin again and watch for signs of over sexualization. Not appropriate and I would be livid. good luck.
YOu have to keep the child away from his cousin since his cousin might use him as a sex toy and tell your son why its not okay to watch porn movies
if the cousin is the same age, I wouldn't worry too much about it. Yes, talk to his parents. Talk to ur son too, explain its not appropriate for him to be watching at his age. I wouldn't keep them apart, they are kids and kids are curious. I wouldn't let them be alone together for awhile though$ I wouldn't worry about him being used as a sex toy either, unless the cousin is a lot older.
absolutly be aware this is not okay an older child/teen and a younger child my answer is NO way you allow this ,yes keep them apart or be in the room when they are together , protect your son ...