Polyamorous Topic

I've been in a new relationship for only 4 weeks and recently, even before the relationship, I've known I am polyamorous. I don't know how to tell my boyfriend..I'm 99% sure he won't agree with it and scared he will give me the ultimatum to either break up or convince myself that I am not poly. This is my first real relationship in 5 years(literally) and he is an amazing guy! Also my first relationship since discovering the polyamorous part of myself. I'm truly happy with him already and I don't want to potentially ruin it, but I feel like I'm hiding part of myself from him and he might react even more negatively if I hide it and tell him further down the road. I could really use some advice or inspiring words right now. My past morals and what I was taught growing up is making me feel like a horrible person and girlfriend.