Our children reflect ourselves. Make this about YOU and the solution will come. If you make it about him, you are playing judge, (Matthew 7:1-2).
You are obviously intrigued by the occult now or once were yourself and this is YOUR exploration.
There is a part of us that is intrigued with dark mysteries. There is nothing wrong with that actually, since GOD Himself Is shrouded in the thickest and most mysterious 'darkness' Himself, (Psalm 18:11), even though He Is Light, (1 John 1:5). There is no actual darkness in Him, only surrounding Him. He does this for our protection, since we cannot begin to handle the glory of His full Presence. Yet, this is the definition of 'occult,' which implies anything hidden behind darkness and mystery.
Scripture tells us that Jesus Himself Is the ultimate 'occult' Figure in the sense that He Is GOD's 'mystery,' embodying and supplying all accessible forms of knowledge and wisdom in a familiar human form, (Colossians 2:2-4).
So even the 'dark' culture is being stolen by evil, which is attempting to co-opt it as its own. The 'Gothic' culture was originally Christian and its mysteries were also Christian. So those living out its subculture and lifestyles are not far from the Christian 'mysteries,' kept from experiencing them only by a deception, being influenced by evil into pursuing a poor imitation that reveals nothing of value and ultimately harms the person.
Are you willing to admit to your own more healthy fascination and pursue the genuine and legitimate version inspired by Christ through His Holy Spirit, or are you content to play 'judge' while your son goes on following the poor examples of those around him instead?
That is the challenge of GOD's personal divine calling for you. You don't have to know how to do it, only recognize that this is what GOD Is asking of you and inviting you to do. We each face a similar challenge in life, which GOD tends to reveal to us through the human dramas around us, especially those involving loved ones. The Holy Spirit Is there to guide us, equip us and help us complete that calling.
6"For I am confident of this very thing, that He who began a good work in you will perfect it until the day of Christ Jesus." (Philippians 1:6).