Good question. Hopefully your fianc? and her ex have a cordial relationship for their child. It would be nice for him to meet you. I mean if you had an ex and you all had a child and your ex was engaged, wouldn't you feel better if you got to meet the guy who would be spending some time with your child? That would be excellent.
Yes. A good understanding, helps He needs to see what type of man will his son spend time with. Be good to that little guy. He will be like a son to you. If u Ever have to correct him. Rember to love him with the other hand. Keep a good balance. :)