I feed my pups Canidae All life stages chicken and rice. I used to feed Roo Royal Canin and she hated the stuff. At first she ate it well but then went off it completely so I switched and she's still not a huge eater but definitely seems to like it better!
Royal Canin is expensive for no good reason I think- the ingredients are still not great. Here's a review:
http://www.dogfoodanalysis.com/dog_f...cat/all/page/1
Canidae is a 5-star food and Innova a 6-star- so I'd try one of those! Good luck.
PS have you tried hand feeding the pup rather than syringe feeding?