Most local authorities will also have either a strategic director for CYP (children and young people) or a Director.
Write one letter to the Chief Executive Officer and you can copy in your MP. That way they will all have the one letter and know each other have it.
Once the CEO gets it, he/she will most likely liaise with the Director of CYP.
It would go down to the specific HOS and they will be asking what and why things were done and looking into whether procedures were followed.
Most importantly you should be asking why when there was an alternative (grandparents), was your DS placed with his father, who has abused him and this was known by the SW.
Emphasise that as it stands, you worry the abuse is continuing because of [list your reasons] and that your DS is so reluctant to return to his dad on visiting you.
Safeguarding child protection and abuse are words they cannot ignore. The letter should not be about you seeing your DS, because they'll say it's not a safeguarding matter and one for either CAFCASS or solicitors to support you with. It needs to be about his safety/abuse/safeguarding concerns.
They may wonder why you've gone to the top...state that you have tried to speak to the SW or whoever you spoke to (name the person in the letter) and were told the case is closed, but you continue to have concerns.