I hope you have reported ALL of this to your attorney. This is NOT acceptable at all for a young child and needs to be addressed by the Courts IMMEDIATELY ie SUPERVISED VISITS by an 'uninvolved' 3rd party.
You are a good mom, never doubt that. Now is the time to use the legal system to help you. Get your lovely son into his counselor right away, even if it is not a scheduled appointment and then the counselor can also write a 'letter to the court' on how this affected DS.
I am so sorry that young man had to endure their ABUSE and yes it was abuse.
See how soon your attorney can get this before the court.
Know that we are walking with you in spirit.
Love and hugs,