That must have been a scary thing to hear...but on the other hand it's good that your BF voiced what he was feeling. I would recommed you either discuss it with him (his feelings about being a father, why he thinks his son doesn't like him etc.) or get him to talk to someone else about it - obviously he needs to express the anger he feels in some way, and quite obviously this would be talking with a mature adult, and not taking it out on an innocent child who really has no clue about what he's doing.
Do his comments scare you simply because their content, or because you get the feeling he *might* some day do something even though you can't really imagine it happening?