Don't know where to go from here with my stepson

During the last 3 months, my stepson has started refusing to come to our house. He won't tell my husband why, but when his mother calls she says it's because he says he doesn't like me.

My husband and I have been married for three years. I have always treated my stepson like my two other kids. The ex tells my husband that he needs to spend time with his son and daughter (which is mine and my husband's) but to not bring me along.

I feel like the mother is making the son resent me because she is angry that her ex has moved on and started a family which does include my stepson. What can or should I do?