Jealous child!

Ok I'm not sure if I'm over reacting but here it goes. My fiancé has a son and I have two other children which know how to share and isn't jealouse of my fiancé. His son is the only child right now but I'm due March 11th and I'm worried because his son is very jealouse of me and hates me and treats me not very good he's very disrespectful towards me and my fiancé says I'm the childish one! I've talked to his multiple time I don't know what else to do! Any advice