Call CPS

There is no reaon where logic does not exist

Call the proper authorities whoever they may be.

"To the ass, or the sow, their own offspring appears the fairest in creation."

Because unless/until they get involved, keeping the kid from the other parent is going to just get YOU in trouble.

~ Anne ~

"Love, having no geography, knows no boundaries."
(Truman Capote)

Lovers do not finally meet somewhere, they were with each other all along.

Keeping the child from visitation based on what you know alone is going to get you in trouble with the courts, you need documentation from an outside source like CPS. Not to mention someone needs to send some help for the other children being abused.

I am pretty sure it is a LAW that if you know a child is being abused in any way you MUST report it to the proper authorities.
Furthermore, there is the moral imperative. No decent person could live knowing that small defenseless children are being abused. You MUST do all that you can to save them.
As for sending another child to their home? If you are CERTAIN that they are being abused, you will probably have to show proof that yo have done someting about it, and then file for an emergency custody thingie. Forget what its called, but if BF fears for the safety of his child he should at LEAST go the proper channels to prevent the child from being around that behavior. That means court.
By the by, enough instances of him filing for emergent custody and winning could eventually turn the standing custody arrangement around and give you and BF custody.
GOOD LUCK!!!