I take it your SO is bipolar? Is he seeing a psychiatrist? If so, would it be possible for you to go with him to the psychiatrist and explain your concerns?
Is he on medication? If not, it really sounds like he should be.

In my opinion, but I'm frequently wrong, if he's bipolar there's not much you can do other than make sure he's getting help.
You need to take care of yourself and keep your kids safe.