I would write to the HOS of the particular section in your local authority.
If you want people to move...I'd go one further and write to the Chief Executive and the Strategic director of Children's services.
Any such letter that a chief exec gets...means things get investigated. Make sure you state safeguarding concerns and summarise the historic abuse from father to son, yet he was still placed there.
State that you would welcome the opportunity to come in and meet with a senior manager to discuss the matter further. Keep mentioning safeguarding, your concerns and how distressed your DS is when he has to leave you.
They will not ignore your written word. Send the letter via recorded delivery.
Find the name of the chief executive and the name of the strategic director, so you address the letters by their names.
My experience (from work) is when such letters are received, questions are asked.
If you send a copy to your local MP as well... believe me they'll be running round to deal with this matter.
If you want to run the letter by me, I'm happy to help ensure the keywords are in there. You can PM me... just blank out names and personal details.