My partner won't tell her son about us we have been together 3 years?

My partner has a 15 year old son and says she will tell him when he is 18 we have been together 3 years an I have to pretend we are not together I can go near her when he is around she has done so much for me I don't know what to do his father is majorly religious evangelical and works for the church so she won't tell him and I can't tell anyone either as she has asked me not to what should I do it's been 3 years and I am finding it hard to cope now???