Being a step mom need advise

Ok hey everyone . So I been with my fiancé for over a year and he has a young son. Every time he comes home from his mother he won't talk to me or let me help him or anything .. She's a lot younger then me and she will just dump him off with anyone so she can go take inappropriate pictures of herself so she can out them on Facebook ... My fiancé says I'm taken this to personal that it should not bother me that my step son is so mean to me but I just can't take it anymore someone help me !