K.O.
It is my total opinion that your fiance should not be disciplining your children at all at this point. It should all go through you. You really need to allow your children to choose whether or not they are going to trust him. You can't force them to trust him, which is what they are going to have to do in order to what to be guided or whatever by him.
I also would not allow anyone into my life that would not respect me enough to not treat my children with much more respect than that. They are going through a major change and they are naturally NOT going to like it. Your fiance really needs to be soft and work into gaining a relationship with them before he is allowed to discipline them.
I know this is very hard because you love someone that is actually willing to accept you and your children, but don't be too desperate to have that man for yourself at your children's expense. A good man will have more patience.
I really wish you all the luck in the world. It is hard to start over with children. Just talk with him and also talk with your son. Ask questions and get him excited about your finace entering your family. He won't be excited if he already feels like he is going to be a threat.
Good luck!