No custody agreement - Options?

My sons mother and I do not have a custody agreement. I was hoping we could work things out amicably but she doesn't seem interested. She has refused to let me take him to my house (about 2 hours from where they live) and I have tolerated it for over a month. She seems to think she is the custodial parent. But my nieces birthday is this weekend and she really wants him there but his mother is saying no. I gave her 12 days notice about the party and have given her half a dozen options as to how to handle it, including offering to pay for a hotel for her if she wants to come also. What are my options? I live in Colorado if that matters.