I rather agree with the advice provided by everyone - up until this point:
If you really loved him, you would accept his past and be cordial to this child, since there is the potential that you may be a step mom.
We humans often have conflicting emotions - about lots of things. Having conflicting emotions doesn't invalidate either emotion (as would be suggested by 'if you really loved him')...and putting a person seeking help, on a guilt trip because they are human, flawed (like all of us - it's just that our flaws vary from person to person), & subject to conflicting emotions doesn't really achieve anything in my view.
The question is - which emotion is going to win out? As a note - fears can be very destructive to relationships, and you are obviously quite fearful in relation to this. Your boyfriend is doing exactly the right thing by his child...with where you are at emotionally - how do you see your contribution to this relationship going? Are you going to be able to handle it? And do you run the risk of trying to damage your boyfriends relationship with his child's mother - which may then traumatise the child?
My thoughts are that, at this stage, it would be a rather bad idea.