Preemie baby bottles/nipples

Hi all,

My preemie was born at 35 weeks. Got out of the hospital 3 days ago. He's now 37 weeks. I bought a couple of bottles from Medela and Philips Avent, but he won't drink from them. He sucks on the nipple but nothing comes out. I tried switching the nipple to a faster flow but it still doesn't work. So far the only bottles that work for him are the one used at the hospital (enfamil) and Gerber, which is very similar to enfamil.

Any one has a preemie who had a hard time drinking from bottles, and which brand worked for you ??

Thanks !!