forgot to ask if you are using a bottle or a syringe to feed him ? if a syringe be very careful not to get it in his lungs , i always use a bottle ,the teats the bottles come with arn't very good they are to hard , if you go to your vet they should have kitten teats , chose the softest ones , i put a hole in the tip with a needle that i have held over heat , make the hole big enough so that when its tipped up it drips out with no hassle

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if you haven't had him to the vet yet i would check inside his mouth and make sure he doesn't have a cleft pallet .