I agree with Jim. This is YOUR choice, nobody else's. Whatever decision you make will change your life forever, and you'll probably always wonder what the opposite choice might have brought into your life.
You sound to me like an intelligent, stable human being who would be able to cope with whatever choice - many children grow up with only one parent, and you can never tell at this stage how he or she might feel about having a stepfather. Non-biological parents can be just as wonderful, if not more wonderful, than the biological ones! (Wow, I make them sound like washing powder!)
On the other hand, this will change your life for good. A child is absolutely for life. You can't get six months down the line and decide you don't want him after all, or a year, or two years. Don't worry about whether or not he'll ever have a father. Just think about whether or not you're ready to raise him. I don't think there's a right or wrong choice here. You can only know where you are now, and make the best judgement about what to do from here.
And I think whatever decision you make, you'll have the emotional intelligence to cope.