Hello,
If you do decide to break up I really like this link, it also has it's own link to a page written specially for kids, for them to read, answering their questions in an age appropriate way,
www.parents.familieschange.ca/telling-the-children/explaining-why-parents-separate-or-divorceI am really sorry that your son sometimes feels scared of your bf, that must be very hard for you. It is good that he has been able to tell you how he feels.
I remember from another post that you have had to intervene in your bf's treatment of his own son, all of this makes considering a break up very understandable.
How difficult will it be in practical terms for you to break up ?