You should sit down and think through the issues very carefully. I think your boyfriend sounds like a loving and honorable man who has a genuine commitment to doing the right thing by his son. I believe it is great that he faces up to his responsibilities. I guess that he'd be equally committed to you and any children you may have in the future. That's a good thing, isn't it?
I don't understand why you'd be scared. Unless you are the jealous type and don't like the thought of anyone else having your boyfriend's attention. If you can't feel warm and caring towards the son, there's no way you should be marrying the father. You'll make your life, the guy's life, and the son's life pretty miserable.
Don't marry the guy if you believe you'll be jealous of the son. In the long run, I think it would be better for all of you if you walk away now instead of waiting until your relationships become even more complicated.