When to marry or not to marry?

My boyfriend and I have been together for 6 going on 7 years. We have finally set a date for our wedding when we acquired his 20 year old son. Threatening to commit suicide, we have taken him into our home. For the last 3 months his son has proven to be the laziest excuse for a person that I have ever met. I am at my wits end. My fiance does not have a relationship with his son since he lived with his mom most of his life. My fiance does not want to be a hard ass therefore continues to cut him slack and allows him to be complacent. This is wrecking our relationship. I cannot take it anymore. I am almost to my breaking point and want out. With less than 4 weeks to the wedding I want to cancel it. I have told my fiance how I am feeling and he refuses to choose. Accommodations and arrangements have already been made and family soon to be on their way. Anticipating the continuance of my unhappy situation with my fiance and his stepson, I am disappointed and hurt. I have a 10 year old that I need to care for and refuse to be unhappy in my own home. Am I being selfish for making my fiance choose? For cancelling our wedding?