A.S.
This child needs to have no contact with his bio mother ever again and you need to get him into a psychologist immediately,perhaps even in a boys ranch.
I know first hand what volatile children are capable of, and your baby is in danger. Your husband needs to see this. The more he ignores it, the worse it is going to get.
Write down everything he has done, or withheld when the baby is going into a dangerous situation how he sits back and watches and show your husband the incidents.
Tell him you only bring this up because you love your stepson and want him to be happy and to provide the most healthy and loving environment possible and the only way that is possible is with intervention.