All of you have to remember there is a child involved here. Maybe you, your boyfriend and the mother of his child should ALL get together and sort this out. She obviously feels threatened by your relationship, maybe she's jealous and hurt still or possibly is having alot of trouble accepting that you will be around her son. Either way, this woman WILL be in your lives forever because of their child together.
Are they divorced? Or was she a girlfriend and then they broke up? Were you part of the picture during their relationship or were they over before you two met?