Fiancé son hates me!

My fiancé ex girlfriend recently told him that their son absolutely hate my guts! And that maybe it's something he senses about my "vibe" and how he should get rid of me. My fiancé freaked out & ask why and were did this animosity came from since he's with us he seems ok. I buy him toys, we play around, and I thought we were building a bond that I wasn't trying to force upon my fiancé son because I am in fact a child of a broken home & I can relate to the pressure that he may feel coming from the same environment but what hurted the most is that  fiancé ex said because their son is 7 is capable of making decisions on his own now and her son said he didn't like me on numerous occasion for no reason (her exact words) maybe he should break thing off with me for his son comfort. My fiancé wants to have a heart to heart talk with his son however his son mother doesn't want him to because she said he gets heartbroken easily and take it the wrong way. I need some advice on this situation. Many times I feel uncomfortable around his son because I'm not allowed to play with him his mother says- since his time is limited with his son she doesn't want me around him. This is really frustrated because I love his son dearly and knew him since he was 1 and now everything changed, we use to always play and do everything together when I seen him and it wasn't one on one time with his dad. The mother said she never said anything negative about me so his reason for saying it, isn't because of her...but I don't know what to believe. She resents me extremely because she still loves my fiancé she recently showed their son pictures of his parents kissing and hugging before he was born. I don't know how to feel or what to do but it's really bothering me now.