Husband cheating

So my husband and i have been together for almost 5 years we have a 3 year old and im currently 31 weeks pregnant with our 2nd child. I have been wondering whats going on with him hes distant wont show affection towards me anymore or anything unless its sex. He has no issues asking for that everyday. So i went to his fb and saw messages from his hr lady and him. He flirts with her all the time tells her she is beautiful and that he wants to take her out. She doesnt flirt back but does clock him in and out when hes not there or running late. Plus when he was injured at work she made it so he wouldnt be drug tested because she covers for him and knew he wouldnt pass. She knows he is married and im pregnant as ive talked to her several times. I guess im asking for advice on how to leave peacefully. He has a violent past and has destroyed my house before throwing stuff.