Hi, congratulations on your DS! I had this problem when my DS was around the same age. I felt so guilty about having to use formula but I couldnt keep up with expressing (he was big but not as big as your DS :-)). But it was only for a few days, he was demand breast fed on coming home and I didnt give him formula again (bf for 15 months). I wish I hadnt worried so much about a bit of formula, it really didnt make any difference, my advice would just be to accept a bit of formula for however long he needs it. I think you will find he feeds a lot when you get him home but it will be so much easier than him being in special care. Good luck!
Don't sweat about a few formula feeds. The nurses in the SCBU where DD spent 10 days had a running joke about the minuscule amount I produced. DD was nearly 9 pounds so ther wasn't a lot to do but top up with formula. As long as you keep regularly expressing, your milk should keep coming.
I found there was a big difference between a feeding baby and expressing. I always produced more with the baby on the breast rather than the machine.
I've done three breast feeds so far and the one just now has actually emptied my boob! I have to go home tonight but from tomorrow I'm able to stay until Sunday so hopefully we'll have it sorted by then! Thanks for all the supportive messages sometimes that's all you need