I would get your syringe ready and then pet him as if you were just having a cuddling session.
Rabbits have a gap in between there teeth (see picture). That will be were you put the this link was very helpful to me after neutering:
Syringe Feeding - Tips & Tricks - Blog
I also smear banana on the tip of my syringe to make it taste better.
This probably won’t not work for you, because he isn’t eating but when I had to feed Pain killers after my rabbits neuter I squeezed it into some banana and then gave the banana to him.
Syringe feeding might be stressful for you both (it certainly was for me) but it’s for the best. Good luck and I hope he gets better