I agree about previous posters about your children because that was me and my brother- my mother was widowed when i was a baby and she spent her life trying to make it up to us- she was our friend not a parent and we were absolute spoiled disrespectful brats. At 18 my mom decided to become a parent and put a stop to it all- it took awhile for us to realize that she was serious and because of it I have become a well adjusted adult and my children benefit from that change no matter how late it may have been.
However, as a former child in a single parent home I have to make a few points- as I have seen it with my moms bf's.
1. if your are afraid he will run- then most likely you are not confident in your relationship yet. and that means he should not be moving in. (and usually if you are scared he will run- he will. I saw this tons of times)
2. your children will most likely rebuff him as a father figure - PERIOD. unfortunately very few children take a new spouse very well but if you as the mother and the "neutral party" lay down "rights" of both parties (the children and the boyfriend) things will go smoother. My mothers boyfriends would try to parent us like 2 days after moving in and we would tell them to take a hike and make their life miserable. however the one boyfriend that we both loved didn't try to be our dad - he told us what to do when it was necessary but it was related to the home- keeping it safe, clean etc but he didn't decide our punishments, priveledges etc that was reserved for our parent-my mom. He is still my friend they broke up over 12 years ago and i still am in contact with him- he was the only father figure i ever had that wanted to be there- the rest we were a by product of my mom.
I know this is long- but my mothers relationships affected us more then she would like to ever admit and I wish she had seen what was happening in a lot of them. I also wish she would have realized that homes do not adjust to new additions overnight, its like when you bring a new baby home some kids adjust overnight others over time and a new spouse brings the same reaction.
Good luck! sorry so long!