It honestly depends on the type of food you're feeding how much you need to get into him. A cat that is overweight that suddenly stops taking on calories is at risk for liver problems (hepatic lipidosis) caused by the sudden mobilization of fat from his body that floods the liver. Keeping nutrition going in is important. A fat cat that doesn't eat for a week will run into problems. A cat that has reduced intake for a couple days won't. I'm sure he's sore today. The worst is day 2 and 3 after any injury like that, but within a week he should feel much better. If the break was back far enough that there's no wound in the mouth (i.e. it's behind where the teeth end), infection shouldn't be an issue.
If you are syringing food to him in a watery form and you're getting in as much as he should eat in a day, there's enough water in there to keep him hydrated. You'll have to look at the can of cat food and see what the daily recommended feeding amount is for your cat (keeping in mind they usually recommend high, so I'd go with trying to get at least 2/3 of what they say into him daily). Break up feeding into several times a day, whatever your schedule allows. Keep offering him water, he'll figure out how to lap it, especially when the swelling is going down after a few days.