I’m guessing here, but I think your vet gave him a painkiller with a duration of 24 hours. That he ate after he had the painkiller makes it extremely likely that pain stopped him from eating – if it were something else, he’d not have eaten even after the painkiller.
So that’s good. We can’t know if the pain will return after 24 hours or if it perhaps was gas pain that made him refuse food and that’s all over now. To me it sounds likely that it had something to do with his teeth – bunnies can have very sensitive gums. But I can’t of course be certain. Low temperature is very common in rabbits that have stopped eating. They sometimes need warming up, in your knee (with your own body’s warmth) or with handwarmers or a snugglesafe or hot water bottle – just make sure he can move away from the heat if he wants to.
If he stops eating again after 24 hours, you should call your vet and ask for a prescription for a painkiller. Metacam (meloxicam) is one that is frequently used with rabbits, and many older rabbits are treated long term with it because it’s an anti-inflammatory as well and helps against arthritis and other joint problems that are common in the elderly. Painkillers lower the body temp though, so keep that in mind – he might need extra warmth.
It can be difficult to feed a bunny Critical Care by syringe, some buns will eat it off a plate or a spoon, esp if you mix it with something yummy like baby fruit-or veg pure or canned plain pumpkin. The best thing is of course if he eats by himself, but if he doesn’t eat enough, you will have to syringe-feed him. If you have someone who can help you hold him in his/&her lap while you work the syringe, it’s a lot easier than if you try to do it yourself. The bunny can be wrapped in a towel, “burrito”-style. You insert the tip of the syringe at one side of the front teeth, there’s a gap there, and you push the food into the bun’s mouth slowly, making sure he swallows properly. He mustn’t get any of the food in his lungs.
12 years is an admirable age for a bunny. Thank you for taking such great care of your boy!