My Boyfriends Sons Mother

I've been dating this man for a year he has a son & I have a daughter. Our kids love each other. His son loves me a lot & I love him.  I love his son like if he was mine, but his mother hates me for no reason she doesn't want me around her son. Every time I'm around she starts to argue with him, causing him to argue with me. I told him to tell her that if she had a problem with me I would love to meet her & maybe fix her issue with me but she refused. I'm constantly helping him financially with his son but she has no idea. She has him on child support, but he still buys him what ever he needs. I don't know why she's acting like this but she's making my life hell.. What should I do in this situation??