Should we intervene? Is this wrong?

By wrong, I don't mean the homosexuality part. If you're just here to say that's wrong than move on. My son is 17 and bisexual and he's gotten close with a new cousin... So how this cousin fits into the family is he's my cousins step son but technically my cousin isn't married to my step cousins mom but they've been together 3 years. We met him 5 months ago and since then my son and him have gotten... Close. My son's name is Kiefer and my step cousins name is Zack and he will ride on Zach's back and say Ziefer coming through! They've been caught cuddling and stuff and idk how to feel. I feel it's inappropriate. My husband said it's not like they're step Brothers who have been raised together. And my cousin is my 3rd cousin who I never see and only now see him because he moved close by and Zach isn't blood. They weren't raise as cousins, they aren't blood and my cousin isn't even married to his mom.... But it still feels incestuous

Edit: My step cousin is 18 almost 19