You want to place the syringe at the back of their lips. Their big teeth are in the front and you could crack a tooth.
I HIGHLY suggest buying the Syringe Mixes that they use for bunnies. I always keep a pack in my freezer. 40cc's a day of the stuff will keep your rabbit healthy, and it's extremely important to syringe feed a rabbit who is in pain and stops eating- or any rabbit not eating for that matter.
As someone else said. A bunny burrito is almost always neccessary unless you have handled rabbits a lot and can slip him into a trance on his back and then syringe feed him when he comes out of it content and comfortable (still on his back). This is for rabbits who can easily be handled and like the taste of the Syringe Mixes. I've only seen one rabbit in person who would do this.
make sure you make your bunny burrito correctly, if you are not pinching the back of the burrito hard enough then the rabbit has a bit of room to kick his back legs and he can hurt himself. As long as you keep that burrito tight then they shouldn't be able to move, and they will eventually accept what you are trying to do with a little bit of resistance. They are smart animals and quickly figure out the faster they deal with whatever big human man is trying to do to them, the faster they can go eat hay in their litter box and lay flat in their favorite spot.