Hello @Anastasia
1) Allow areas of privacy for your son as he grows into his manhood. It may be hard, but it might help him become more resourceful
I have been in similar situation as @Faith-and-Hope as I have not been able to block the serious toxic person's influence (his half sister) over my son. Her dominance and troublemaking occurred from before his birth and succeeded in destroying my relationship with my biological daughter, so that is also hugely problematic and triggering for me.
She proved her recent toxicity to my son, and also gossip behind the scenes about him, by calling police in pseudo welfare check. She has to come to terms with mental illness in her own biological parentage without blaming me for everything, when I was the one who did all the work. My son had kept giving her chances over all his life, and hoped she would be positive in his life, even I was ambivalent as I hoped half sister would be better than no sister, and accepted and allowed their contact, even though inside I was screaming with worry.. She has proved her toxicity now, and my son has developed other relationships outside of my knowledge, which is fine, This month he has told me he no longer believes the family is genuine family for him except for me. We have been very isolated and maybe I made the mistake of keeping open and reaching out and hoping. He now says he has accepted a friend I have not met yet, as a sister, and is developing socially, better late than never, and he is testing about trust and values..
@Anastasia I believe VALUES are the key concepts to communicate with your son to enable him to become aware of the quality of relationships of all kinds, friend, romantic, sexual .... Try not to micromanage his truth telling or saying a lie, as he is of an age where he needs some privacy from his mother. I feel for you, but his honest understanding is really worth it, un the long run. Perfect communication all the time may be unrealistic. What to do with MUMMA BEAR. idk. PRAY!