I don't think that he hates them, but he doesn't know what to do with them.
My husband had two kids from a prev marriage and is 10 years older than me, when we started dating he seemed to forget that I did not have children and lived the life of a single girl that was free to do anything I wanted, he actually moved in with me 1 month after we started dating then moved his 7 year old son in with us one month after that...lol at first I was like sure this is fun...haha then I was like
ahahah what the hell did I just do I went from no kids to one kid full time????? it was pure hell. Me and my now husband talked it out and we decided that we had to slow it down that yes I accept the fact he has children from his prev life, but that I'm not ready to be a mom or half mom to a 3 and 7 year old it was just too much.
We slowed down and it got a better, now we have a daughter together and another on the way, his other children are great at being siblings and love my little girl so much, I care deeply for them as well but to tell the truth I have still found it difficult to completely love them.(no haters let me explain)
I have heard alot of people say that the step parent souldn't complain about hurt feelings just because they are the adult and the kids are just being kids but for example
: you can not honestly tell yourself that you will not be hurt when you help your step daughter make something and have her tell you from start to finish that she's making it for you, only to have her finish then look you in the face and say you know I'm going to give this to my mom instead, but its okay because you saw me make it though.... I know that kinda sounds lame but multiply that by 3 years now and her doing it everytime
we make or do a project together, I know that she doesn't mean to hurt my feelings but after so many times you kind of shut them off so that you no longer care if they do something for you or not.... I hope that kind of makes sense and shows how I kind of had to halt my heart at a certain point or I would have fallen into depression, your fiance may be just trying to protect himself, please don't kid yourself being a step parent is not a fun job in the least you get all of the grief and none of the reward, but as long as you and your partner keep your communication open it will get better but honestly communication is key with this.
remember he may never be able to love them till far in the future but he will grow to like them very much, and that is a good thing