I think that eventually it will peter out on it's own. But the kid needs some therapy and people telling him it is fine to let out those fears. Right now he is six and the only people he knows strong enough to fight that fear is WWE Tag Teams. So he needs to have a safe space to be angry, upset, and a little violent. This is a perfectly acceptable way to let out those fears.

However, if you parents plan to put him into gymnastics, acting, and continue on the wrestling route... you might consider putting him into a martial arts that proclaims a peaceful approach to fighting, like Ju Jitsu.

My brother got his ass handed to him by bullies all the time. He was defeated by nine and felt he had no respect for himself. So my mom put him into Ju Jitsu and he learned to defend himself, have pride in who he was, and be a confident entity. It also helped that he had a real, solid rolemodel who lived that life.

It seems your cousin wants some sort of power and control over his situation. Putting him in an activity that calls for self-respect and self-control might work well to balance out the aggression of WWE.