I am the custodial parent but my son wants to live with his dad (NCP) which I have allowed but do not agree to the living arrangements and the situation. I was told I can make him come back (he is only 15) but I have doubts I do not want my son to be affected. I am not sure how to handle this situation I was told if he stays there 6 months then he can take possession of him.

You don't take possession of children - they aren't furniture.

There is no automatic transfer of Court-ordered custody (there appears to be a custody order) based on a time frame. If his father WANTS to be the custodial parent the fact that you have allowed the child to live with him for X months outside the Court Order can and probably will be mentioned during the custody proceeding.

What do you mean by you "do not want [your] son to be affected"? Affected by what? A custody fight? The circumstances where he is now? Returning to you?