Why is my baby finding it so hard to feed?

Hi, my son is now 9 weeks old and I have been feeding him expressed breast milk from a bottle since birth (had a C section and really struggled to get him to latch on). He's never been a bit eater, but it seems to be getting progressively worse as the weeks go by. At the moment, I'm struggling to get more than 2-3 oz in him per feed (around 15-20 oz per day), and he's sick 1-2 times per day. Doctors don't seem concerned as he's putting on weight, albeit very slowly, and he's not dehydrated. However I know it doesn't take much for the wee things to become dehydrated, so am constantly worried that he's 'on the edge', so to speak.

I've tried him on formula a couple of times, but he has been sick both those times. There is also no correlation between what time of the day he is sick, or whether he is lying down or upright. I'm getting really frustrated, as I feel he isn't getting enough and it means I'm constantly worried about it. 

Does anyone else have any experience of this, and any advice they can give? It's so distressing when he's sick (projectile out of nose and mouth) and refusing feeds. Despite what the doctors tell me, I can't help feeling there's something wrong...