None of my babies have been planned! I've been over-the-moon which each (slightly more apprehensive about this one, but happy nonetheless) and DH has had a different reaction to each. He was happy with our first, but felt a little overwhelmed because it was so *new*. We were both in our early 20s, living the party/young lifestyle, weren't financially stable, etc. But he stepped right up to the plate and never looked back after becoming a youngish dad. He had a negative reaction to our second, elements of which are still so painful for me I can't even type them out. But you know what? He came around really fast and became really excited about the baby when I was about 4 months along. He was very happy about this last little one, even though it's pretty much the worst timing possible!
I've heard from so many mama friends that even if the initial reaction isn't too positive, dudes come around :). You guys are planning to make a commitment to one another, you're in love, and I think you'll be just fine. I really feel that it's never an ideal time to have a baby so when surprises happen, and you wish to continue the pregnancy, it's a bonus because you don't have the stress of trying to plan a *perfect* time to conceive.
And of course, CONGRATULATIONS!!!