Try holding him standing up like he's on his legs in one hand with your thumb on his beard side facing you. Press your thumb against his beard and slide it down toward his neck. The friction from your thumb against his beard should pull his mouth open so you can feed him via syringe. Offer the syringe from the side of his mouth and go slowly with small bits at a time so he doesn't inhale any food by accident. I've found that wrapping him burrito style in a small towel may help you to hold him still.