Hi ~ I agree with riotgrrrl -- it's great you're supporting your fiance' thru this. Yes, it's going to be a bit difficult for awhile, but once you get to know this little guy I'm sure you'll fall in love with him and welcome him into your home.
It WILL be hardest on your fiance' because all of a sudden he will be a Dad, and that's a pretty big responsibility to have tossed at you immediately!

I'm sure his head is spinning right now. He's going to have child-support as well as learning how to be a Dad. You are going to have to help him with this -- he isn't going to know HOW to be a Dad, and hopefully you can help him learn.
With time, I'm sure this will work out just fine. This little guy needs a Dad, and as long as your fiance' WANTS to be a dad, things will be ok. I wish you the very best. God bless and take care. Hugs, Lee