E.,
Congratulations on your great first pregnancy and beautiful experience as a mother! (I HATE YOU, too!) ;)
It's unfortunate that your partner is not intersted in having another child, (He already has his 2- right?) I believe that you should have the opportunity to have your 2 if you really want it. However, if you do it, your relationship may not survive. It seems to me that you're more than capable of being a good, loving single-mother, so that's not a worry, but your baby(ies) would potentially be happier having two loving parents.
Another thing, I often feel that the lives of only-children are not as full or happy as they could be. In the event that your partner won't change his mind, what do you think is the better situation for you little one? Having a sibling, palymate, and companion and one parent or having two doting parents?
I remember not too long ago, I had a professor who was caring for her elderly father and often and matter-of-factly stated how she didn't have anyone but him since she'd been an only child, never had children of her own, and was now divorced- It always made me sad for her, even though she seemed to have come to terms with her life. Your little one could potentially be like her because even though she has a half-brother, he's so much older.
I'm sure you already heard this as well, but every pregnancy is different. I hope if you do it, that the second will be just like your first one, but please keep in mind that they can be as different as night and day! My first and second were both pretty good, but the third was very stressful due to all the "medical" possibilities/probabilities/tests/etc. In the end everything was great, however.
Anyway, I wish you the best! I know it is an extremely difficult decision because it will affect everyone in your family.
-I.