Does my stepson have a problem or is it normal?

My stepson has been taking my lingerie and hiding it in his room for 10 years now, and I have been with his Dad for 12 years. He has received counseling for this, but it continues and he is now approaching 18. Could he develop a serious fetish that might get him in trouble as an adult, or is this just a simple young man exploring his sexuality and it will fade away in future years? I'm scared for this kid, but his dad says it's just a norman part of growing up. Any suggestions on how to handle this?