this situation is common….from experience I would advise you to act with much care.
firstly try to put yourself in your child’s shoes-
I strongly believe the child does NOT think lets test my parents nor does he think I want to be bad.
the child does not view it wrong maybe a heart to heart talk explaining what you feel as apparent and giving reasons not related to rabbonim don’t allow its totally assur, since the child has heard it all before and it does not make an impact rather it just flows out his ear’s.
by giving over how you feel..you can then come to a compromise such as a filter..give back for lock up time …or buying films that are frum and jewish…a child cannot be to restricted as it may lead to a bad ending.
you want a frum child to follow your ways maybe let loose a little, to provent major problems.