This doesn't sound like typical pre-teen sexual experimentation. Not by a long way.
The issue to me is that Sal is obtaining sexual gratification from exploiting weaker or helpless creatures.
From where I sit, Sal is sick and needs help. He is too sick for you to help him, or your husband, or your mother in law. He needs intensive professional help. And I don't mean to be negative but the success rate with kids as disturbed as Sal seems to be, isn't great. I agree that he needs an expert or as close to it as you can get. His behavior is truly disturbing. As much as you might not want to see him go back to live with his mother that might be what is needed to protect your baby son and your pets.
His grandmother doesn't sound like a viable option as he will continue to act out with her, from what you say.
I am so sorry this is happening to you. You MUST protect your baby son from him.
