I think you have rose colored glasses on. I know it's hard to see, when you are so close to the situation. If your fiancée had nothing to hide, he would be hiding nothing. Have you got personal knowledge that you have seen and heard with your own eyes that your bf's boss is a woman hater?. or is this what your boyfriend has told you? Be honest with yourself now. Did he tell you this is how his boss was first, and then you formed conclusions second? If his boss is this way, then your boyfriend should have taken this up directly with his boss, and confronted him with handing out his phone number. Ask yourself this. What would your bf's boss have to gain by medling in your relationship? It just doesn't make sense. If he was a woman hater... in your words... why would he give another female his number? One and one don't equal three. It just doesn't add up.
Your fiancée is very defensive. Men in relationships that get defensive are almost always covering for something, and trying to justify their actions. He told you that she is one of the prettiest girls he's ever seen? Hello! Big flashing red flag. Although I question the girls mothers' intentions for feeling the need to call you, I'm sure she's not phycho. She is likely upset that a man that is engaged with 3 children, is calling her 17 yr. old daughter. There is definitely something going on between your fiancée and this girl. No doubt about it. This is as clear as the nose on your face, but you are too close to see it for what it is. Actually, I digress, you already know. You just don't want to accept it yet. You know what your gut is telling you, but you are trying to explain it away. Go with your gut, it is seldom wrong!
There are just too many red flags here. He is NOT telling you the whole truth at the very least. I know it's hard, but this is a bad relationship.