Feeling so hurt.. he doesn’t think it’s his baby

My fiancé and I just got engaged and had our first ultrasound.. the next day he started acting a little off.. then he voiced his concern about it not being his baby. I’m extremely hurt. I’ve never cheated on him and I’m worried he’s just looking for an out.. we were on and off the month I conceived but it’s 100% his baby and I just feel like my world has shattered. He wants a paternity test but how am I supposed to be with him that long knowing that’s what he thinks about me? Idk what to do or say guys.. how am I supposed to marry him now