First of all, great job so far! You've given him a great start by giving him pumped milk from the start, and you're seeking help to continue, instead of just giving up. The other moms are correct, breast milk is best but it is not the only.
Having said that, it's important to realize that supplementing, over time, creates a cycle that will reduce your breastmilk supply and create the need for more supplementation. If that's what ped. said, though, definitely take his advice over mine. However, if you really want to breastfeed, and it's really important to you to give that to your son (and preemies especially need it!), then I would shop around for a more breastfeeding friendly pediatrician.
If you do continue to supplement with formula, please consider offering him the AVENT bottle. The large nipples on the AVENT seem to mimic the breast. Then when you put the avent nipple in his mouth, try to get as much of the nipple in as you can; with breastfeeding, they need to take in a large amount of breast tissue to properly latch, and you want to try to mimic that as closely as possible. His lips should go all the way, or almost all the way to the white collar.
Also mimicking breastfeeding, you might want to take off your shirt and bra when you give him a bottle, and tuck his arm behind you and get him real close to your skin, just as he would be positioned on the breast. Most people hold the baby away from them when they bottle feed.
Also, you should be pumping each time he gets a bottle, so that your body senses a demand at the right times, for the right number of times.
You could drip a little formula on your nipple (is there a way you could try it without the shield for a few times?)right before feeding so that he tastes the familiar formula and thinks it's a bottle. That might "trick" him into breastfeeding enough times for him to get used to, and even prefer, the breast.
If you're using a pacifier, lose it. Be his "human pacifier" for a few weeks. Meaning, put him on the breast even when you don't think he's hungry. Even if he doesn't eat, this method will help signal your body to make lots of milk, and might make up for the lack of demand from the formula feedings. It will be a sacrifice of YOU time for a short while, but if your ultimate goal is to get him breastfeeding then this will likely help. You can always give him back the pacifier once breastfeeding is well established.
Carry him around. A lot. Get a front carrier and wear him as much as you can take. Hold him, more than you think is normal for a mom to hold a baby. Exaggerate it. This will help with the bond that might be slightly severed from the extended hospital stay.
Get lots of good healthy food, and good healthy rest. I know, easier said than done! But if you're going to be the one taking care of the baby, then you need to make sure you are taking care of yourself first. The housework can wait, but this is your only year (or less or more) to do this with him, and it's a short year! So sleep, eat, drink lots of water. Cuddle baby Ethan. Enjoy him. Breastfeed as much as you can; if that's only once a day, remind yourself that that's better than none at all. Let yourself and Ethan learn how to breastfeed, but don't let it consume you to the point that it causes emotional damage.
Just my input! As with any advice, you take the parts that you want, and ignore the parts you don't want. :)
H.